Simon Pagenaud has the fastest practice time ahead of the Honda Indy Toronto.
Pagenaud went around the 11-turn, 2.81-kilometre street course at Exhibition Place in one minute 0.0084 seconds.
IndyCar points leader Helio Castroneves was second in 1:00.0218, while defending champion Scott Dixon followed in 1:00.0769.
James Hinchcliffe of Oakville, Ont., was fifth in 1:00.0937.
The session ended under a red flag after Ryan Briscoe stalled on the track. Practice was also delayed after Castroneves stopped with an electrical issue.
Josef Newgarden set the fastest time in the opening practice on Friday morning.
Newgarden went around the 11-turn, 2.81-kilometre street course at Exhibition Place in 1:00.2385 seconds.
Will Power followed in second in 1:00.4497, while James Hinchcliffe was third in 1:00.5728.
Hinchcliffe was delayed getting on track with a cooling issue.
Toronto's Paul Tracy is the last Canadian to win the event in 2003.
Castroneves entered the Toronto races first overall in 2013 only to eventually cede the title to Scott Dixon.
But this could be Castroneves' year. The 39-year-old Brazilian yearns for a championship.
"More than anything, my friend," said Castroneves. "More than anything."
Castroneves lost by just 27 points to Dixon last year after being undone by a disastrous doubleheader in Houston where Dixon finished first and second, respectively.
He also settled for second behind Sam Hornish Jr. in 2002, his first full season in the Indy Racing League after four seasons in Champ Car. Both series merged in 2008.
Power has come even closer.
The 33-year-old Australian finished second overall every year between 2010 and 2012. Power could taste the title in 2012, but he crashed at Fontana. His team scrambled to get him back on track, but Ryan Hunter-Reay edged Power out in a dramatic season finale.
He doesn't want another year of being IndyCar's bridesmaid.
"It's going to be a tight race to the end," Power said. "But man, I'm doing everything I can.
"I'm determined to win the thing. Really determined."
'Pretty bad hit'
The Penske cars, always a formidable presence in the series, have been consistently dominant this season as the team looks to end an eight-year title drought.
Power began the season on a tear. He won the season opener at St. Petersburg and again at one of the Detroit races. He was on the podium five times and never finished out of the top-10 in the first eight races.
But penalties have hampered Power's season. He's been charged five times so far, some more costly than others. At the Grand Prix of Indianapolis he hit pit equipment and was dropped out of contention. Later at Pocono on July 6 he was charged with blocking Castroneves.
"The only one that cost me was the one at Pocono," said Power. "Double points, man that was like actually a 40-point swing right there, which is [a] pretty bad hit."
Power's last four results have also been poor — 14th and 11th at Houston, 10th at Pocono and 14th in Iowa. He nearly won the second race at Houston but had to pit with a broken suspension just two laps from the finish. Power has two career wins at Toronto in 2007 and 2010.
'This place owns me'
Castroneves, who took over the overall lead last Sunday at Iowa with an eighth-place finish, has just one win so far after splitting the Detroit doubleheader with Power.
But Castroneves has been consistent. He's been on pole twice, stepped onto the podium five times and has nine top-10 finishes in 12 races so far.
Last year Castroneves, who has never won in Toronto, made a breakthrough with a second-place finish in the second race behind Dixon.
"I feel this place owns me," said Castroneves. "We start on the pole, finish second, finish all over the place. ... This place always helps a lot in terms of strategy. Some people in the back sometimes take chances and they end up paying off, but it all depends.
"We can only try to predict the unpredicted, which is hopefully start in the front, stay in the front if the race goes that way and then collect a lot of points."
'Thinking every session'
Points are becoming ever-more precious as the season winds down. Six races remain ahead of the Aug. 30 finale at Fontana, and at least 100 could be won by a driver who sweeps the 85-lap, 2.81-kilometre street course at Exhibition Place.
There's also plenty of time for another driver to steal the title from Castroneves and Power. Hunter-Reay, who has three wins including one at Iowa, trails Castroneves by just 32 points. Pagenaud and Penske's Juan Pablo Montoya are also within 100 points of first place.
Power and Castroneves both say they're trying to balance a focus on racing with an awareness of the standings.
"You're just thinking every session," said Power. "You're thinking about getting the car right and having good weekends.
"Not even that, every session you're thinking about, every lap. Doing it right, that's it."
'Got to push through'
Castroneves added it's hard to not get overwhelmed by the numbers.
"Sounds not a lot when you say six races to go," he said. "But with points involved, it's a lot and with that aspect, not that I don't look ahead.
"But you've got to push through to make sure that you finish ahead of those important [drivers]. You've got to actually save for a rainy day in case that something bad happens, you have enough to cover for the last race."
It might just take the last race for the championship to be decided.