In the end, the decisive threat came not from Castroneves but from Hunter-Reay, who blasted his way past Castroneves first and then took the lead itself from Sato on lap 199 with seemingly no problem whatsoever. Sato came in for his final pit stop next time by, still on a slightly earlier sequence than the new leader but more significantly his tyres showing the evidence of the torture they had received while battling through the backmarkers.
As the race entered its final 50 laps, Sebastian Saavedra brushed the wall before pitting on lap 202, but it was Dale Coyne Racing's Ana Beatriz spinning and hitting the wall in turn 4 on lap 212 that finally brought out the fourth and final caution of the day. The timing was a disaster for Sato, who had already pitted, because it meant that the current leaders were able to make their final stops under yellow. That left Sato at the back of the lead lap at the restart, with Hunter-Reay ahead of Castroneves, Power, Viso, Dixon and Hinchcliffe as the green came out on lap 220 with 30 laps remaining.
The end now in sight meant everyone was now in full attack mode to the finish and there were several white-knuckle battles going on throughout the field - except at the front, which defending race champion Ryan Hunter-Reay imperiously had under lock and key from there to the chequered flag by several seconds from Castroneves. It makes him the first back-to-back winner at Milwaukee since Tony Kanaan achieved it in 2006/7, and he's also just the second two-time race winner in the 2013 IZOD IndyCar Series season so far along with his team mate James Hinchcliffe.
"The team did just awesome today," said Hunter-Reay. " We had varying levels of grip through the whole race, different levels of balance, and we just stuck with it. In the end, we knew what we had to do to win. What a race it was. It was a lot of fun."
Helio Castroneves hung on to the second spot despite strong pressure from his Penske team mate Will Power in the closing moments - pressure that Castroneves chopped off with a fierce pinch that showed there was absolutely no question of team orders going on here, with the pair lucky not to end up in assured mutual destruction. Remarkably it's Power's first podium of the year after nine of the 19 races of the season completed.
"That was close," said Power of his late-race scrap with his team mate for second position. "I wanted to race him very cleanly because he's leading the championship. I was like 'Man, I don't want to take you out.'
"Still, a good result for Team Penske. Helio continues to get good points, and it's cool to get the #12 Verizon car up on the podium," Power added. "We've just got to keep chipping away and in the meantime, I'm going to help out Helio as much as I can. If I can beat him, I will and make some hay on the championship."
"Certainly it was a great day for us," contributed Castroneves. "We were a little conservative, and we had to be patient ... It's not a win, but you know what, I'm very, very happy."
Penske's success means that the two surviving Andretti Autosport cars - EJ Viso and James Hinchcliffe - ended up being pushed off the podium and had to settle for fourth and fifth instead, ahead of Scott Dixon who was the best of the Honda runners in sixth place for Ganassi.
"Any time you can get a top-five finish, especially with how competitive this field is, it's a good day for the GoDaddy car," said Hinchcliffe. "I feel like I spent half the day just trying to get around the lap traffic!"