Kyle Kirkwood topped the first practice session at the Honda Indy Toronto ahead of his Andretti team mate Romain Grosjean.
Swedish driver Felix Rosenqvist ended up in third place ahead of Colton Herta and Scott McLaughlin at the bumpy Exhibition Place venue.
At the start of first practice in the Canadian city, many cars joined the circuit and completed an installation lap in the one-hour 15-minute session.
A J Foyt driver Benjamin Pedersen stayed out on the track and set a 1:23.8745 lap time before returning to the pit lane.
After a brief lull, a flurry of cars joined the track, including New Zealander Marcus Armstrong, who drove to a 1:03.8989 ahead of Sting Ray Robb.
With an hour remaining, Armstrong held the fastest time ahead of Robb, with Pederson in third.
Rinus VeeKay slotted into second place for Ed Carpenter Racing behind Armstrong as the track continued to rubber in.
In his first IndyCar event, Tom Blomqvist, replacing Simon Pagenaud for the Honda Indy Toronto, gained valuable track time for Meyer Shank Racing.
The times continued to tumble as Christian Lundgaard usurped Armstrong atop the timesheets with Ryan Hunter-Reay, his closest challenger.
Rahal Letterman Lanigan showed good speed early in first practice, as Jack Harvey went to P1 with a 1:03.5120, creating an RLL 1-2.
Rosenqvist momentarily held the quickest time for Arrow McLaren SP before Devlin DeFrancesco achieved a 1:03.1161 at his home race to go quickest in front of Callum Ilott.
With 40 minutes remaining, Herta broke the 1:03 barrier, setting a 1:02.4045 ahead of Graham Rahal as a flurry of quick times were posted.
The brutal bumps of the Exhibition Place circuit provided an extra challenge for IndyCar’s 27 drivers.
Alex Palou’s 1:02.2554 was good enough to go quickest ahead of the six-time series champion Scott Dixon with just over half an hour to go.
Some drivers complained about the violent bumps on the track in Toronto as Kirkwood regained the quickest time with a 1:01.8639 in front of American driver Alex Rossi.
As the session drew to a close, the Andretti driver continued to hold the quickest time with 15 minutes to go as the drivers got to grips with the tricky 1.755-mile track.
Lundgaard, the first driver on the softer, green sidewall alternate tyres, usurped the American with a 1:01.6996.
Palou bolted on the Alternate tyres with just over 10 minutes left on the clock for Chip Ganassi and went to P1 at the track he scored a sixth-place finish last year.
As Kirkwood retook the top spot, Lundgaard suffered a big slide through Turn 10, but kept his number 45 car out of the concrete wall.
After a quiet session, Romain Grosjean improved to first position with a 1:00.8575 as his Andretti team mate Herta sat in second.
Agustin Canapino ran into the runoff area at Turn 3, but managed to continue to three minutes to go.
Arrow McLaren found themselves with two cars in the top five, with Rosenqvist in third and Rossi in fifth.
Kirkwood snatched top spot in first practice away from Grosjean, setting a 1:00.8075, with second Practice and Qualifying on Saturday.
