The two-time series champion lapped the 11-turn, 2.1-mile temporary street course in 1m16.4042s but ended his session by smashing into the wall at Turn 1.
After a rain shower that impacted the preceding Indy NXT practice, the track had dried completely for the 75-minute session.
Chip Ganassi Racing’s rookie Marcus Armstrong set the early pace at 1m18.2776s, less than a tenth clear of Toronto winner Christian Lundgaard (Rahal Letterman Lanigan Racing).
Lundgaard then improved to 1m18.009s to take P1 away, 0.02s ahead of Romain Grosjean (Andretti Autosport).
Arrow McLaren’s Pato O’Ward was an early visitor to the Turn 9 run-off but rejoined to take the top spot at the halfway point with 1m17.4437s. Series leader Alex Palou then took over at the top for Ganassi with 1m16.9063s, 0.06s quicker than Colton Herta (Andretti).
Teams ran the green-sidewalled alternate rubber inside the final 15 minutes after their race-run sims were completed on primaries.
Palou produced 1m16.6494s to extend his advantage, before Power toppled him with 1m16.4042s. Alexander Rossi (McLaren) went P3 just before a red flag for Callum Ilott (Juncos Hollinger Racing) becoming stranded in the Turn 11 run-off.
The session restarted but was red flagged again when Power slammed the wall at Turn 1 and was forced into an awkward rejoin, while Benjamin Pedersen stalled at Turn 11. “Not ideal,” rued Power.
O’Ward was fourth, ahead of Lundgaard, Herta, David Malukas (Dale Coyne Racing), Grosjean, Scott McLaughlin (Penske) and Scott Dixon (Ganassi).
Series debutant Linus Lundqvist was a hugely impressive 11th for Meyer Shank Racing, as he fills in for the still-sidelined Simon Pagenaud.
Armstrong, Rossi and Jack Harvey (Rahal) were others to visit the Turn 9 run-off, while McLaughlin also had a moment that caused him to come to a halt at Turn 10. Marcus Ericsson also slithered to a halt, this time at Turn 4, and struggled to the 25th fastest time.