Scott Dixon was frustrated by a pitstop call at Detroit that restricted him to fifth place in the race and kept his title rival Helio Castroneves in the hunt going into this weekend's finale at Chicago.
Dixon led from Castroneves in the early stages of the race but his Chip Ganassi team called him into the pits during a caution period on lap 17, while Castroneves and the other front-runners stayed out.
That dropped the Kiwi to 18th place and he lost so much time in traffic that the first four cars were able to pit under green 13 laps later and still rejoin ahead of him.
Dixon remained in fifth for the rest of the race and only came out with a 30-point championship lead when Castroneves was forced to hand the lead to Justin Wilson after blocking the Briton.
Dixon was frustrated to finish fifth when he felt that he could have won the race.
"It was pretty pitiful," he said. "The car was clearly quicker than anybody's and we were saving a ton of fuel up front and having a merry old day.
"It's just a hard situation when you're in that point and nobody else pits and you have a lot of slower guys that have a lot of fuel and you have to try to pass them. It's very tough to dive in there when you're fighting for a championship.
"It's not a pretty picture when the leaders go and pull out a big lead, pit, get fresher tyres and more fuel than you and come out ahead. It's pretty frustrating."
But Dixon stood by his Ganassi team, despite their call costing him the victory.
"Nobody was going to touch us and still we ended up fifth, so it's disappointing, losing like that is tough to deal with.
"I didn't decide it, but we win as a team and we lose as a team. My anger is just from frustration."
Dixon now lies 30 points ahead of Castroneves going into to the final race and just needs to finish eighth at Chicago to win the championship.