Scott Dixon sealed a fifth IndyCar title at Sonoma after Alexander Rossi's early error proved insurmountable for the Andretti driver, whose team-mate Ryan Hunter-Reay was victorious in the season finale.
Hunter-Reay followed up his first pole since 2014 with a lights-to-flag victory in which he was largely unopposed as the title race took the driving seat.
Dixon closed rapidly around midway into the race but backed out before Hunter-Reay took a measured win.
Hunter-Reay took the two bonus points for leading the most laps just after half-distance, ruling out the already very slim title hopes of Penske duo Josef Newgarden and Will Power - who needed to win and hope for a Dixon disaster alongside those extra points.
But Newgarden and Power's slim hopes faded as the former stalled in the pitlane, while the latter made no impression on the leaders and finished third.
Dixon had entered the race with a 29-point buffer over his true title rival Rossi, meaning the Andretti Autosport driver had to at least finish several places ahead of the title favourite.
But Rossi was forced into a fightback after clumsily clipping the rear of team-mate Marco Andretti at the first corner of the race, damaging the front-right corner of his car.
He slowed, cutting a section of the track, and returned to the pits before resuming the race a lap down.
Rossi took until lap 35 of 85 to get close to unlapping himself as he gave title rival Dixon something to think about by hounding the Ganassi man.
A slight error from Dixon allowed Rossi through before he finally unlapped himself when Hunter-Reay made a stop, and Rossi was then handed a brilliant stroke of luck when Rahal Letterman Lanigan Racing's Graham Rahal stopped and caused a full-course caution just past half-distance.
Rossi had pitted just before the yellow, putting him at a fuel and tyre advantage.
He quickly dived back into the pits from 12th for a splash of fuel and new red-walled tyres, planning a short stint aimed at regaining track position.
The Andretti driver managed it in stunning fashion as he made up five places from 13th in the two laps following the restart and continued to carve his way back up the field - making contact with Newgarden in the process - and got to seventh before his final pitstop.
But a final finishing position of seventh was not enough to prevent Dixon's fifth title, as Rossi lost sixth to Sebastien Bourdais on the final lap as his tyre performance dropped off.
Marco Andretti and Simon Pagenaud had both looked like podium contenders early in the race but instead finished fourth and fifth ahead of Bourdais and Newgarden.
Indy Lights champion Patricio O'Ward starred on Saturday by making the Fast Six pole shootout on his IndyCar debut but endured a messy first stint and struggled to match the tyre life targets set by Harding Racing.
Despite this, O'Ward put in a strong performance after his first stint to finish ninth ahead of Ganassi's Ed Jones, Santino Ferrucci and Tony Kanaan in his 300th Indycar race.
Jordan King was 13th in his best IndyCar performance to date, climbing from 25th on the grid and making up more places than anyone else in the field.
Two drivers celebrating new contracts in Spencer Pigot and Takuma Sato retired from the race with mechanical problems, with the latter having a spectacular plume of smoke following his RLLR-run machine before mid-distance.
|1||Ryan Hunter-Reay||Andretti Autosport||Dallara/Honda||85||2h02m19.1667s|
|2||Scott Dixon||Chip Ganassi Racing||Dallara/Honda||85||2.7573s|
|3||Will Power||Team Penske||Dallara/Chevrolet||85||3.6550s|
|4||Simon Pagenaud||Team Penske||Dallara/Chevrolet||85||4.6306s|
|5||Marco Andretti||Andretti Herta||Dallara/Honda||85||19.8030s|
|6||Sebastien Bourdais||Dale Coyne Racing||Dallara/Honda||85||21.6393s|
|7||Alexander Rossi||Andretti Autosport||Dallara/Honda||85||28.3778s|
|8||Josef Newgarden||Team Penske||Dallara/Chevrolet||85||34.6288s|
|9||Patricio O'Ward||Harding Racing||Dallara/Chevrolet||85||42.8662s|
|10||Ed Jones||Chip Ganassi Racing||Dallara/Honda||85||44.2254s|
|11||Santino Ferrucci||Dale Coyne Racing||Dallara/Honda||85||44.6273s|
|12||Tony Kanaan||AJ Foyt Enterprises||Dallara/Chevrolet||85||1m00.4138s|
|13||Jordan King||Ed Carpenter Racing||Dallara/Chevrolet||85||1m04.0637s|
|14||Zach Veach||Andretti Autosport||Dallara/Honda||85||1m05.4899s|
|15||James Hinchcliffe||Schmidt Peterson Motorsports||Dallara/Honda||85||1m12.8026s|
|16||Pietro Fittipaldi||Dale Coyne Racing||Dallara/Honda||85||1m14.2459s|
|17||Jack Harvey||Meyer Shank Racing||Dallara/Honda||85||1m15.6462s|
|18||Carlos Munoz||Schmidt Peterson Motorsports||Dallara/Honda||85||1m18.6345s|
|19||Matheus Leist||AJ Foyt Enterprises||Dallara/Chevrolet||85||1m22.3819s|
|20||Colton Herta||Harding Racing||Dallara/Chevrolet||85||1m23.4673s|
|21||Max Chilton||Carlin||Dallara/Chevrolet||84||1 Lap|
|22||Charlie Kimball||Carlin||Dallara/Chevrolet||76||9 Laps|
|23||Graham Rahal||Rahal Letterman Lanigan||Dallara/Honda||66||19 Laps|
|24||Spencer Pigot||Ed Carpenter Racing||Dallara/Chevrolet||38||Mechanical|
|25||Takuma Sato||Rahal Letterman Lanigan||Dallara/Honda||15||Mechanical|