The American duo both carded six-under-par 64s so sit atop the leaderboard at the Royal Montreal Golf Course - the lowest first round on record in Quebec.
Putnam and Petrovic are a shot clear of compatriot Kyle Stanley and Canadian amateur Taylor Pendrith at the PGA Tour event.
Starting on the back nine, Putnam - yet to finish in the top 10 this season - was comfortable throughout as he went blemish-free on the opening day, carding six birdies.
Four of those birdies came during his last nine holes in a late flurry for the 31-year-old.
"I hit a lot of fairways and I hit a lot of greens," said Putnam.
"It got windy there this afternoon and it was tough golfing but somehow I was able to keep it in the fairway and take advantage of the soft greens, get a couple shots close and make a couple of five-to-10 foot putts."
Fellow American Petrovic also had a bogey-free round, holing four birdies on the front nine before capping the day with an eagle at the 12th hole.
As for Stanley, he had five birdies at the 10th, 12th, 14th, 15th and 17th holes as he hit a 65 in the first round.
Local favourite Pendrith sunk seven birdies and two bogeys in a solid start to proceedings.
Australian pair Greg Chalmers and Robert Allenby headline a group of 11 players tied for fifth position at four under par.
Chalmers and Allenby, who both started on the back nine, are two shots off the pace alongside Justin Hicks, Charlie Wi, Nick Watney, Troy Merritt, Bo van Pelt, Dicky Pride, Charl Schwartzel, Jason Bohn and Joel Dahmen.
Meanwhile, defending champion Brandt Snedeker is five shots behind the leaders after his round of 69 that included a double-bogey and two bogeys.