Colton Herta led fellow IndyCar rookies Felix Rosenqvist and Santino Ferrucci in opening practice for the Laguna Seca finale, despite all three going off and Herta causing a red flag.
Herta's 1m10.734s lap in his Harding Steinbrenner Racing Dallara-Honda pipped Rookie of the Year points leader Rosenqvist of Chip Ganassi Racing by just 0.0719 seconds.
But in the dying seconds of the session Herta looped into the runoff on the outside of the initial left-hand part of the Corkscrew, and no amount of wheelspinning could recover the situation without outside assistance.
That red was the second of the session, as Tony Kanaan had earlier binned his AJ Foyt Racing Dallara-Chevrolet at the final corner.
Rosenqvist had a long run through the sand on the outside of the Turn 4 right-hander and a wild moment through the Corkscrew before scoring his best time on his final lap, while Dale Coyne Racing driver Ferrucci also had some wild moments before setting a time just 0.15s off Herta's best.
Team Penske's Will Power was fastest of the series veterans and fastest of the Chevrolet runners, albeit 0.27s off the ultimate pace, in fourth.
Zach Veach was fifth and fastest of the Andretti Autosport cars, while Simon Pagenaud was first of the championship contenders in sixth.
The other title contenders - points leader Josef Newgarden, Alexander Rossi and Scott Dixon - were ninth, 24th and 14th respectively.
Rossi was on a hot lap when Kanaan brought out the red flag, and he then asked for a significant set-up change that required him to step out of the car for 20 minutes.
When Rossi resumed, there were just three minutes to go, at which point Herta's spin prevented him setting a representative time.
Practice one times
|1||Colton Herta||Harding Racing||Dallara/Honda||1m10.7335s||-||13|
|2||Felix Rosenqvist||Chip Ganassi Racing||Dallara/Honda||1m10.8054s||0.0719s||18|
|3||Santino Ferrucci||Dale Coyne Racing||Dallara/Honda||1m10.8837s||0.1502s||16|
|4||Will Power||Team Penske||Dallara/Chevrolet||1m11.0024s||0.2689s||12|
|5||Zach Veach||Andretti Autosport||Dallara/Honda||1m11.0816s||0.3481s||13|
|6||Simon Pagenaud||Team Penske||Dallara/Chevrolet||1m11.1519s||0.4184s||12|
|7||Sebastien Bourdais||Dale Coyne Racing||Dallara/Honda||1m11.1581s||0.4246s||10|
|8||Spencer Pigot||Ed Carpenter Racing||Dallara/Chevrolet||1m11.2382s||0.5047s||9|
|9||Josef Newgarden||Team Penske||Dallara/Chevrolet||1m11.2460s||0.5125s||9|
|10||Matheus Leist||AJ Foyt Enterprises||Dallara/Chevrolet||1m11.2702s||0.5367s||11|
|11||Marco Andretti||Andretti Herta||Dallara/Honda||1m11.2827s||0.5492s||8|
|12||Marcus Ericsson||Schmidt Peterson Motorsports||Dallara/Honda||1m11.3015s||0.5680s||13|
|14||Scott Dixon||Chip Ganassi Racing||Dallara/Honda||1m11.4451s||0.7116s||8|
|15||Ryan Hunter-Reay||Andretti Autosport||Dallara/Honda||1m11.4532s||0.7197s||8|
|17||Tony Kanaan||AJ Foyt Enterprises||Dallara/Chevrolet||1m11.6237s||0.8902s||5|
|18||Takuma Sato||Rahal Letterman Lanigan||Dallara/Honda||1m11.6773s||0.9438s||12|
|19||Jack Harvey||Meyer Shank Racing||Dallara/Honda||1m11.7449s||1.0114s||10|
|20||Conor Daly||Andretti Autosport||Dallara/Honda||1m11.9320s||1.1985s||10|
|21||Graham Rahal||Rahal Letterman Lanigan||Dallara/Honda||1m12.0043s||1.2708s||9|
|22||James Hinchcliffe||Schmidt Peterson Motorsports||Dallara/Honda||1m12.0125s||1.2790s||10|
|23||Ed Jones||Ed Carpenter Racing||Dallara/Chevrolet||1m12.1601s||1.4266s||13|
|24||Alexander Rossi||Andretti Autosport||Dallara/Honda||1m12.9932s||2.2597s||4|