Charley Hoffman rolled in six birdies in a seven-under 65 on Saturday to take a one-stroke lead into the final day at the Canadian Open.
Hoffman was one ahead of fellow American Kevin Chappell (66), who bogeyed the last hole after finding water, with Gary Woodland and Robert Garrigus a further stroke behind.
Garrigus's round of 62 tied a course record.
Hoffman, who is 17-under overall, said staying aggressive and making birdies was key to winning the tournament, which would be his first victory since his Texas Open win last year.
"Making pars on Sunday out here is not going to get me anywhere," he said in a television interview after the third round.
"I'm going to keep my same game-plan -- a bunch of drivers, hit a bunch of wedge shots close and hopefully make some putts."
Low wind, receptive greens and plenty of sunshine led to a shootout at Glen Abbey Golf Club in Oakville, with 30 players boasting overall scores of 10-under or better heading into Sunday.
World number one Dustin Johnson appeared poised to take the lead until his second shot on the 11th hole landed in the water, leading to a double-bogey.
He finished the day with a 68 and sits five strokes behind Hoffman.
"I just hit a terrible shot," said Johnson, who also misjudged several putts including a birdie chance at the 18th.
"I missed quite a few putts today but I feel like I'm rolling it well. I just need to go out tomorrow and see a few go into the hole," he said.
Former world number one and 2004 winner Vijay Singh failed to continue his sizzling play from Friday, shooting a 71 that had him tied with seven other players at 11-under.
The 54-year-old is seeking to become the oldest winner in PGA Tour history.