Nicky Hayden has put one hand on the 2006 MotoGP World Championship by repeating his debut US GP victory of one year ago - while Valentino Rossi suffered a potentially title-shattering retirement.

Hayden, starting from sixth on the grid, gained three places into turn one - and was soon hot on the heels of fellow American Kenny Roberts Jr and pole sitter Chris Vermeulen.

But Vermeulen grabbed vital breathing room by bravely diving inside Roberts at the final turn, to lead at the end of lap 1 of 32. Rizla Suzuki's star rookie, a double Laguna winner in World Superbikes, then rode the wheels off his GSV-R to pull a near two-second lead by the time Hayden squeezed past Roberts at the top of the corkscrew on lap 9.

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Questions remained about how long Vermeulen's Bridgestones could perform in the crippling temperatures - of 39 degrees C (102.2 degrees F) ambient and 56 degrees C (132.8 degrees F) on the track - but with nearly a third of the race gone the Aussie was showing no signs of weakness and Hayden had a straight fight on his hands.

The Kentuckian chipped down Vermeulen's lead relentlessly over the following laps and - as the halfway stage arrived - was within striking distance. Chris soaked the relentless pressure for another lap before running slightly off line and reluctantly conceded the lead.

Thereafter, Hayden quickly built a comfortable two second buffer in front of his nearest rival - that then grew steadily to three-seconds by the chequered flag - but it wasn't over Vermeulen...

The #71 had continued to lap consistently after being passed by Hayden, but then began to suffer from an intermittent fuel supply problem and was mercilessly reeled in by Nicky's team-mate - and double 2006 race winner - Dani Pedrosa. The 20-year-old rookie had dropped from fourth to seventh during the opening laps, before starting his fight back from lap 4 onwards.

The Spaniard despatched fellow Honda rider Marco Melandri and front row starter Colin Edwards in as many laps, but then found his progress blocked by determined former 250cc rival Casey Stoner. The pair battled for every turn, catching third placed Roberts in the process - resulting in a dramatic moment on lap 10 when Pedrosa elbowed past Stoner into turn six, then outbraked Roberts at the final corner... only to run wide and lose drive on the exit.

That allowed both Roberts and Stoner back through and the trio then fought side-by-wide for the next half a lap; with the experienced Roberts again losing out to the youngsters as he was shuffled behind both Casey and Dani respectively. But, perhaps due to the constant pressure from Pedrosa, Stoner's challenge finally ended in the turn six gravel when he low-sided his Honda LCR out of third place on lap 15.

By the time Vermeulen lost the lead two laps later, Pedrosa was just 2.7secs behind - and closing fast. With ten laps to go Pedrosa was the fastest man on track, an attribute that resulted in him pulling to within a second of Vermeulen and thereafter it as a case of 'when not if' he found a way past.

The move finally came when Vermeulen, having gamely resisted another factory Honda rider, got off line and was forced to surrender the runner-up spot with eight laps remaining. Once past, Pedrosa was unable to close Hayden's lead and the top two positions were effectively settled.

But the real drama was only just beginning...

World champion Valentino Rossi, just tenth on the grid after another horrendous qualifying, had initially struggled to make much needed progress - remaining tenth for the first three laps and ninth for the next seven. But the Italian jumped past both Suzuki's John Hopkins and Camel Yamaha team-mate Edwards on lap 11, before being promoted to sixth by Stoner's exit.

By the midway point Rossi was still sixth, but had now caught Roberts and Melandri ahead of him - overtaking the American on lap 19, then passing his fellow Italian with the same inside-at-turn-six move a lap later. Although victory was out of the question, The Doctor was expected to quickly catch up to Vermeulen and limit the championship damage with a repeat of his 2005 podium...

Alarm bells in the Rossi camp began ringing when he stopped closing on Chris and - with just five laps to go - suddenly began fading backwards from the Australian. As Roberts and Melandri rocketed past a lap later, a rear tyre problem was identified as the cause of the seven-times world champion's dramatic deterioration.

Rossi kept his composure and looked set to at least nurse his M1 to the flag, when huge clouds of smoke suddenly began spewing from his exhaust! With his engine performance plummeting, the #46 slipped back to eleventh at the end of lap 30, then stopped altogether on the very last lap. By that point Rossi had been black flagged, although it's not clear if that - or total power loss - caused him to halt while out on track.

After clawing back up to within 26 points of Hayden following a nightmare start to the season, it is impossible to understate the significance of Rossi's US GP retirement - which has left him 51 points from Hayden with just six rounds to go. Nevertheless, Vale showed much less emotion then when he was forced to retire from the lead at Le Mans and calmly watched marshals dispose of his powerless M1, before waving to the fans as he was chauffeured back to the pits.

Rossi may have been the highest profile casualty of the last few laps, but another rider who deserved much better than he got was Vermeulen: Just moments after Rossi's M1 began smoking, Vermeulen's growing fuel supply problems reached critical and his bike began cutting out. The young Aussie thus lost a podium to Melandri and then dropped a position to Roberts before reaching the finish line in fifth, one place ahead of American team-mate John Hopkins.

Carlos Checa claimed an impressive seventh on his Dunlop shod M1, with Loris Capirossi the lead Ducati in eighth and Edwards the last of the home riders in ninth.

After a strong qualifying Colin - riding in a special 'stars and stripes' livery - never looked a threat this afternoon while, by total contrast, Hayden had reversed an at times "inconsistent" pace with a faultless race day ride to secure his third ever grand prix victory.

Hayden celebrated by riding the cool down lap with his Bubba Shobert replica helmet and a large American flag in his hand - just as he had one year ago. Hayden's 2006 win wasn't as emotional as his debut triumph, but it looks to have been far more significant.

With Rossi's retirement dropping him to fourth in the standings, Pedrosa has now moved back into second - but an increased 34 points from Hayden - with Melandri a further 10 points back. With only 150 points still up for grabs, Nicky's 2006 title lead - which until now has often looked 'on loan' - now appears permanent as MotoGP heads for its long awaited summer break.

Round twelve of the 2006 world championship will take place at Brno, in the Czech Republic, on August 20.

US Grand Prix:

1. Hayden
2. Pedrosa
3. Melandri
4. Roberts
5. Vermeulen
6. Hopkins
7. Checa
8. Capirossi
9. Edwards
10. Gibernau
11. Tamada
12. de Puniet
13. Ellison
14. Hofmann
15. Elias
16. Cardoso