Scott Dixon claimed victory in Sonoma and stole a last-gasp win in the 2015 Verizon IndyCar Series championship, after a catastrophic misfire between Penske team mates saw Juan Pablo Montoya lose the title in a wins countback following a dead-heat in the points standings.

It's Dixon's third win of the season and his 38th for Ganassi in total, and it meant he had overcome a 47 points deficit to Montoya coming into the double-points season finale to pick up the team's 11th IndyCar drivers title against all the odds.

"There was still a chance and that's what I was hoping for," a stunned Dixon said in victory circle as he tried to get his head around the outcome. "I don't know what to say. This season we had some big races, and this was the biggest. We were such a long shot."

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CLICK: Full results from Sunday afternoon's GoPro Grand Prix at Sonoma Raceway.

Inevitably there was a sombre start to the day's proceedings, with a number of tributes - including a moment's silence and the playing of the British national anthem - made in memory of the late Justin Wilson who died at the start of the week from injuries suffered in last Sunday's race at Pocono Raceway. Eventually however it was time to turn to the matter in hand, and pole sitter Will Power together with Josef Newgarden duly led the field to the green flag under perfect sunny blue skies for the start of the GoPro Grand Prix of Sonoma and the first of the afternoon's 85 laps.

Power leapt away to an immediate resounding lead while Newgarden fended off a surge from Ryan Hunter-Reay. In a big boost for his hopes of sealing the title, Juan Pablo Montoya succeeded in immediately moving up to fourth place ahead of Ganassi's Scott Dixon, while by contrast Montoya's main rival for the championship Graham Rahal was on a set of scuffed tyres and found himself swallowed up at the start, dropping to tenth place before finding his way back around Marco Andretti to begin the fightback.

Another of the championship contenders, Power and Montoya's Penske team mate Helio Castroneves, was in for an early pit stop at the end of lap 3 after sustaining some front wing damage in the opening lap jostling. Also making an early visit to pit lane was KVSH Racing's Sebastien Bourdais two laps later, in this case in pursuit of a strategic advantage down the line, with Tristan Vautier (Dale Coyne Racing), Jack Hawksworth (AJ Foyt Racing) and James Jakes (Schmidt Peterson Motorsport) among those already at the back with little to lose doing likewise. That only paid off if there was a caution before the rest of the field came in for their own stops, and instead the race remained stubbornly caution-free.

Dixon was the first of the leaders to pit on lap 13, earlier than expected and signalling that tyre degradation was a bigger problem that had been anticipated. Sure enough the top two were themselves in next time by, Power's pit stop not going as quickly as hoped for meaning that he was still in the pit box when his team mate Simon Pagenaud arrived and was forced to queue for service. When Power pulled out and Pagenaud moved forward it blocked Newgarden who was forced to drive through the Penske stall (and over the air hose, a technical no-no) to get out and back on the track. Fortunately there were no such hiccups for Montoya when he made his own appearance on pit road next time by.

Ganassi part-timer Sebastian Saavedra and Andretti Autosport pair Marco Andretti and Oriol Servia were the last cars to make their stops so that on lap 23 Power was back in front ahead of Newgarden, Dixon, Hunter-Reay and Montoya with Tony Kanaan in sixth ahead of Bourdais, Rahal and Charlie Kimball as Saavedra came back out in tenth place. However it was already time for those cars that had made really early stops - Castroneves and Bourdais - to now initiate their second visits of the afternoon to pit road.

A lack of cautions so far meant that fuel usage was now becoming almost as big a headache for the teams as tyre degradation, but the yellows were finally out on lap 33 after Newgarden's CFH Racing team mate Luca Filippi slowed on track with throttle issues. The majority of the field pitted but it had worked out well for Saavedra, Andretti and Servia. Kanaan had also pitted just before the caution and now picked up fourth ahead of Bourdais and Castroneves, while among those who came in under the caution it was Dixon who won the race off pit road to rejoin in 13th ahead of Power, Newgarden and Montoya.

The race resumed on lap 39 but the old adage 'cautions breed cautions' immediately applied, and with disastrous consequences for Penske. As Power tried to thread his way through traffic he cut across the front of Montoya: the contact spun Power off the track with damage to the right rear wheel guard of the #1 to bring out a new caution, and there was also damage to Montoya's front wing. Both men headed to pit road for repairs where they were joined by a number of other cars also making additional stops in order to get their fuel topped up. There was no change in the top five, but Dixon was now up to ninth place followed by Newgarden and Rahal, while by contrast Montoya and Power had dropped to 23rd and 24th places for the restart on lap 43 - and suddenly things had gotten very interesting in what until then had been a fairly vanilla race.

With Kanaan on the freshest tyres of the leaders, the Ganassi #10 briskly moved to the lead even before Saavedra, Andretti and Servia came down pit road at the end of lap 48 for their off-sync stops. There were stops too for Sato, Bourdais and Pagenaud and as the race reached lap 50 Dixon found himself up to second place behind Kanaan. With Rahal in fourth and Montoya back in 14th, it meant that for the time being it was the Kiwi in line for the title by three points if the race should stop right now. Of course, in reality the race still had another 35 laps to go and anything could still happen before the matter would actually be decided.

It was looking good for the Kiwi however: Dixon's position was consolidated still further when his team mate Kanaan pitted and handed him the lead ahead of Newgarden, while another of the Ganassi line-up Charlie Kimball made his own contribution to Dixon's title hopes by passing Rahal for third place. "My team mates were phenomenal, they helped all year and we won this together," Dixon said later, acknowledging the input of all his fellow Ganassi drivers throughout 2015 and especially at Sonoma this weekend. "There is no one person that achieved this on their own and I just thank everybody so much."

Now acutely aware of the clear and present danger to his title hopes, Montoya was on the move himself with rapid successive passes on James Jakes and Carlos Munoz to climb to 11th place, but he needed still more over the remaining laps if he were to convert his season-long lead in the standings into a fully realised championship. Instead it proved to be a case of two steps forward and one back for the Colombian as he succumbed to a lovely clean pass from Kanaan who was now one of the fastest cars on the track after his pit stop.

Dixon meanwhile had pulled out a six second lead over Newgarden at the front. The really bad news for his rivals was that Sonoma is a track he's proved several times in the past is a place where he can not only find the lap times but also do it will saving fuel, including last year when he won the race on a similar strategy. It didn't look like he had a weak spot in his armour as the race - and the season - headed to its climax.

By contrast it seemed like the end of the road for the Rahal Letterman Lanigan Racing #15 which was haemorrhaging positions as tyre wear reached such levels that Rahal was unable to break into corners without running off for fear of spinning out entirely. Anything less than a win now wouldn't be enough for Rahal - and he wasn't going to get it.

With 24 laps to go, Newgarden was on pit road from second place for his final scheduled stop of the day - but it proved to be a disaster, the car stalling and refusing to refire, while a flash fuel fire licked over the bodywork until quickly extinguished by the crew. While the CFH crew laboured to get Newgarden back out, Dixon and Rahal were both in a lap later for their own drama-free stops, and then Montoya was in next time but having to rely on the vanishing hope that Dixon wouldn't be able to make it to the finish on fuel.

Dixon had just cycled back to the lead when a full course caution was declared for James Jakes, after brake failure snapped the #7 left into a concrete wall and then the car was gathered up by the tyres forming the chicane at at turn 9. That closed the field back up and allowed those cars that still needed one more pit stop to come in for fuel, but it also meant that any concerns Ganassi might have had about Dixon making it to the finish on his current tank of gas were effectively assuaged. If Montoya was going to win the title now then he would have to do it by making up positions from his current 12th place in order to finish no lower than fifth to overturn Dixon's prospective race win.

The restart on lap 69 lasted only a lap before contact between Hawksworth and Munoz left the Andretti Autosport #26 spun around and perched on the kerbs at turn 7. However, even in that brief spell of running Montoya had managed to work his way up to eighth place - just three places away from the position he needed if he were to successfully stave off Dixon's assault on the title.

Dixon leapt away to safety at the restart, but that was almost beside the point now: it all depended on where Montoya finished, and this time the #2 was unable to make any forward progress on the cars in front and needed to pray for something to break in his favour. Sure enough, it happened - with nine laps to go, Bourdais ran into the back of Rahal and spun the #15 out of sixth place. Montoya picked up a position straight away and then another when Bourdais received the inevitable drive-thru penalty for causing an avoidable contact. That put Montoya into sixth place, but with Dixon having gained the bonus points for leading the most laps of the race in the meantime it meant that the two rivals were now tied on points, and Dixon would win the title based on which of them had the most wins over the course of the season. There was nothing else to it - Montoya needed to get around Schmidt Peterson Motorsports' Ryan Briscoe ahead of him or else hope that someone else would spin out and gift him the extra point he needed.

Briscoe was clearly struggling on the shorter-life red option tyres, and Montoya - set a concrete achievable target - gave chase and soon wiped out the gap between them on the track. However the laps were also counting down and with two laps to go there was still two seconds between the pair - and even once he got up to the back of the SPM car there would still be the significant matter of actually getting past him.

Three minutes later, Scott Dixon took the chequered flag and with it the 100th victory for Chip Ganassi Racing. "I knew the car was strong. We were getting fuel mileage so easy, which was key," he said. "We could roll the car through the corner and obviously get the mileage. You never know until the last lap. That's what it came down to. You hope for it. We had to do our best job and that's what we did today and luckily enough it worked out."

After Dixon came Hunter-Reay, Kimball and Kanaan, and then all eyes turned to who would cross the line in fifth place. It proved to be the gold-and-black livery of the SPM car rather than the red-and-white of the Penske, meaning that Montoya had failed to seal the title by the most excruciating, absolute narrowest of margins.

"We had a good car. We had a good start and we did everything we needed to do at the beginning," Montoya said, ruing the mid-race contact with Power that had almost certainly cost him the title. "Will overshot and I was fighting with Newgarden, we shot the corner, we got inside and he cut across and I was there, and we touched and that was it. We came from behind and do our best, just wasn't enough."

Behind Montoya, his contrite team mate Power crossed the line in seventh ahead of Sato, while a rather brilliant performance from Rodolfo Gonzalez resulted in a ninth place finish for Dale Coyne Racing. Briscoe's SPM team mate Mikhail Aleshin also shone with tenth place in his first IndyCar outing in a year, and behind Marco Andretti there was a 12th place for Oriol Servia in the Andretti Autosport #25 which made for a final deserving tribute of the day to Justin Wilson.

While the drivers championship couldn't have been closer, it was a different situation in the manufacturers standings with Chevrolet having been confirmed as the winners over rivals Honda before the race had even started.

"Our fourth consecutive manufacturers championship is the result of consistent preparation, teamwork and execution by our Chevrolet teams, drivers and technical partners," said Jim Campbell, US vice president of performance vehicles and motorsports for Chevrolet. "Together, we focused on delivering strong engine and aero performance all season long."

The celebrations started on victory lane and will continue for much of the week, after which there is a long off-season before the 2016 season gets underway next March.

See full results from Sunday afternoon's GoPro Grand Prix at Sonoma Raceway.