Petrovic joined Furyk on 10-under 130 with a four-under par 66, while DeLaet, like Furyk, matched the Royal Montreal Golf Club's Blue Course record with a 63 for 132.
The round put DeLaet in a strong position going into the weekend, boosting his chances of ending the 60-year victory drought for Canadian golfers in their home open.
But there are plenty of players in striking distance. DeLaet shared third place on eight-under with American Kyle Stanley, who posted a 67.
Northern Ireland's Graeme McDowell carded a 65 for 133, heading a group that also included American Justin Hicks (67), Scott Brown (66) and Andrew Svoboda (66).
The fireworks started early as Furyk, DeLaet and American Matt Kuchar went out together from the 10th tee and found birdies plentiful.
"It was a great start. All of us were making so many birdies out there," said DeLaet, who finished with nine birdies and two bogeys. "You felt like you had to make putts just to keep up."
Furyk, coming off a fourth-placed finish at the British Open last week, finished with seven birdies -- jump-starting his round with four straight birdies starting at the 11th.
He picked up three more coming in, and after a birdie at the eighth, had a chance to break the course record at his final hole, the ninth, but his 28-foot birdie attempt slid right.
"I think today's round and yesterday's weren't much different -- a lot of fairways, a lot of greens, a lot of opportunities," Furyk said.
"Today I think I capitalized on a few more of those, but the golf course is still playing pretty soft, the greens are very receptive."
DeLaet birdied the tough par-three 17th, but then found the water at 18 and bogeyed.
He bounced back with a birdie at one, and after another bogey at four he birdied three of his last four holes -- including a birdie-birdie finish at eight and nine.
"I felt like I was in pretty good control of my game all day," he said. "I had a couple of hiccups, but I really rolled in a lot of good putts."
The group combined made 22 birdies, Kuchar posting a five-under 65 that put him on 134 -- among a group of eight players sharing ninth place.
"I don't know if I've ever played in a group where we shot 63, 63, 65," Furyk said.
"I was giving Kuch a bunch of crap for being the schlub in the group shooting a 65!"