Scott Dixon took the opening spoils from Toronto, as he ended the first practice session as the fastest driver.
Dixon's fastest lap time around the tight street circuit was a 1:01.0723, which was over one-tenth fastest than Penske's Josef Newgarden, who concluded in second place.
Newgarden emerged from the pits at the end of the session to set his fastest lap and place as the highest Chevrolet.
Newgarden still holds the lead in the championship but saw a decrease in his advantage as Alexander Rossi took his second win of the season last time out.
Dixon's teammate Felix Rosenqvist was third and will be looking to continue building his momentum following a sixth-place finish three weeks ago at Road America.
Following Rosenqvist was a combo of Andretti, Penske, Andretti, Penske, as Alexander Rossi, Simon Pagenaud, Ryan Hunter-Reay and Will Power.
Sebastian Bourdais was eighth, ahead of Takuma Sato and Spencer Pigot who rounded out the top ten.
Sato went for a spin during the session and was almost collected by Marco Andretti as he did so. Andretti ended the practice outing in 11th.