Josef Newgarden narrowly beat Alexander Rossi to victory during a caution-filled wet/dry first IndyCar race of the weekend in Detroit.
Team Penske driver Newgarden was a low-key third through the opening stint but a fortuitously-timed sole pitstop ensured he leapt Rossi, and he held off his Andretti rival through to the chequered flag.
The start of the race was delayed by an hour after a band of thunderstorms swept through the Detroit area, with the distance reduced from 75 laps to 75 minutes.
Rossi retained his pole advantage through a single-file wet start as Scott Dixon passed Newgarden for second in the brief phase before a caution period was declared when Matheus Leist spun through Turn 2.
Rossi preserved a two-second gap to Dixon through the first stint as the circuit gradually dried, with Marco Andretti the first driver to gamble on slicks when he came in one-third of the way through proceedings.
A handful of others made the move a few laps later but it was Newgarden - having briefly dropped to fourth - who was the first frontrunner to stop, moments before a second caution period was called when wet-shod Ed Jones slid into the Turn 7 wall.
Newgarden cycled to the front of the pack as his rivals were forced to pit under caution, with Rossi emerging second and Dixon third - but the reigning champion's race came to an end soon after.
On the second post-restart lap Dixon clipped the inside wall entering Turn 6 and nosed his Ganassi-run car into the exit barriers, sustaining terminal damage in the process.
That caused the third caution period, with a fourth segment needed shortly afterwards when Leist had his second incident of the day as he repeated Jones's error of sliding into the Turn 7 barriers.
By the time the restart signal was given for a fourth and final time it left a 12-minute shootout for victory between Newgarden and Rossi, as they surged clear of the chasing pack.
Newgarden was unable to shake off Rossi but the ex-Formula 1 racer could not mount a serious challenge, clearing the 2017 champion for his first win since St Petersburg's season-opener.
Takuma Sato (Rahal Letterman Lanigan) prevailed in a close fight for third place as he narrowly edged Felix Rosenqvist (Ganassi) and Ryan Hunter-Reay (Andretti).
Indianapolis 500 winner Simon Pagenaud recovered from a lowly grid spot of 13th to take sixth for Team Penske, ahead of Sato's RLLR team-mate Graham Rahal and Andretti's Zach Veach.
Veach caused an additional formation lap when he spun prior to the start but steadily worked his way through the pack.
James Hinchcliffe and Spencer Pigot capped the top 10.
Newgarden's Penske team-mate Will Power had starred during the early stages, rising from 12th to sixth, but his race was undone in the pitlane.
Power was erroneously given the signal to leave his pit box before his front-right wheel was securely attached, and it worked its way loose seconds later; Power recovered for repairs but classified only 18th.
Race one result
|1||Josef Newgarden||Team Penske||Dallara/Chevrolet||43||1h15m30.5932s|
|2||Alexander Rossi||Andretti Autosport||Dallara/Honda||43||0.8237s|
|3||Takuma Sato||Rahal Letterman Lanigan||Dallara/Honda||43||11.4760s|
|4||Felix Rosenqvist||Chip Ganassi Racing||Dallara/Honda||43||11.8833s|
|5||Ryan Hunter-Reay||Andretti Autosport||Dallara/Honda||43||12.2263s|
|6||Simon Pagenaud||Team Penske||Dallara/Chevrolet||43||12.5127s|
|7||Graham Rahal||Rahal Letterman Lanigan||Dallara/Honda||43||13.1515s|
|8||Zach Veach||Andretti Autosport||Dallara/Honda||43||14.0022s|
|9||James Hinchcliffe||Schmidt Peterson Motorsports||Dallara/Honda||43||15.2409s|
|10||Spencer Pigot||Ed Carpenter Racing||Dallara/Chevrolet||43||16.1462s|
|11||Sebastien Bourdais||Dale Coyne Racing||Dallara/Honda||43||16.9309s|
|12||Colton Herta||Harding Racing||Dallara/Honda||43||17.2807s|
|13||Marcus Ericsson||Schmidt Peterson Motorsports||Dallara/Honda||43||17.9085s|
|15||Tony Kanaan||AJ Foyt Enterprises||Dallara/Chevrolet||43||25.6122s|
|16||Marco Andretti||Andretti Herta||Dallara/Honda||43||26.0652s|
|18||Will Power||Team Penske||Dallara/Chevrolet||43||33.0561s|
|19||Santino Ferrucci||Dale Coyne Racing||Dallara/Honda||43||43.1639s|
|20||Ed Jones||Ed Carpenter Racing||Dallara/Chevrolet||42||1 Lap|
|21||Matheus Leist||AJ Foyt Enterprises||Dallara/Chevrolet||30||Contact|
|22||Scott Dixon||Chip Ganassi Racing||Dallara/Honda||23||Contact|