The 47-year-old Petrovic was the last player to get in the field, grabbing a spot when Benjamin Alvarado withdrew Tuesday.
“I didn’t set my expectations high because I got in late, so this was kind of my practice round,” Petrovic said. “I walked the back nine (Wednesday).
Petrovic eagled the par-5 12th hole and had four birdies in his bogey-free round. He won his lone PGA Tour title in New Orleans in 2005.
“I was just trying to hit fairways and greens, trying to see the golf course and see if I could make a few putts,” Petrovic said. “I drove the ball pretty good overall.”
The 31-year-old Putnam had six birdies in a bogey-free round, hitting 11 of 14 fairways and 14 of 18 greens in regulation.
“It was tough ball-striking, but somehow I was able to keep it in the fairway and take advantage of the soft greens to hit a couple of shots close,” Putnam said.
Winless on the PGA Tour, Putnam won twice on the Web.com Tour last year and topped the regular-season money list.
Canadian amateur Taylor Pendrith and former Clemson star Kyle Stanley are a stroke back, and former Masters Tournament champion Charl Schwartzel, Nick Watney and Robert Allenby topped the group at 66.
The 23-year-old Pendrith, making his first PGA Tour start, is coming off his senior season at Kent State.
“I never expected that,” said Pendrith, from Richmond Hill, Ontario. “I was playing good golf coming in here, so I just played golf. Five-under is pretty nice. Everybody has some nerves. If you didn’t have nerves on the first tee something would be wrong with you. But after I played the first hole, I felt calm and felt I should be here.”
Stanley opened with 5-under 30 on the back nine and played the front in even par.
“I love this golf course,” said Stanley, the 2012 Phoenix Open winner. “It’s similar to the one I grew up with in Washington, so I felt pretty comfortable. I hit the ball well on my first nine today. I was able to make a few of the long range putts that got the round going for me.”
Watney had six birdies and two bogeys on the tree-lined layout.
“You’re not playing behind the 8-ball to make the cut,” Watney said. “It’s only one round, but I’d much rather play well in the first round than not.”
Two-time champion Jim Furyk and Canadian David Hearn shot 67s, and Brandt Snedeker, the winner last year at Glen Abbey in Oakville, Ontario, had 69.
“Any day you come off the course and shoot 3-under and feel you left a few out there, you feel it’s a good day,” Hearn said.
Canadian Mike Weir, an eight-time PGA Tour winner and former Masters champion making his 24th start in the event, had 70.
DIVOTS: Dustin Johnson struggled to a 74, and John Daly had a 76. ... Pat Fletcher, born in England, was the last Canadian winner, taking the 1954 event at Point Grey in Vancouver, British Columbia. Carl Keffer is the only Canadian-born champion, winning in 1909 and 1914. Albert Murray, a Canadian also born in England, won in 1908 and 1913.