Scott Piercy profited from a final-hole blunder by William McGirt to snatch a one-stroke victory at the RBC Canadian Open in Ontario.
Piercy held his nerve to two-putt from 50 feet at the last to cap a three-under 67 and take the clubhouse lead on 17 under, but a play-off looked in store as McGirt arrived on the 18th tied for the lead.
But McGirt, who three-putted to bogey the 15th, dumped his approach into a bunker and splashed out to 18 feet before missing his putt for par.
Piercy had started the final day three behind Robert Garrigus, but he surged into contention with four consecutive birdies from the second.
His challenge stalled with bogeys at eight and 14, but another birdie at 15 proved pivotal as McGirt and Garrigus faltered down the stretch.
"I've been playing good for a while now," said a delighted Piercy.
"I felt like I got a good couple breaks and continued to play solid, and I'm kind of speechless, to be honest. I'm really excited to be the champion."
Garrigus could not buy a birdie putt as he opened with 13 straight pars before finally getting one to drop at the 14th, but he three-putted the 16th and two closing pars left him one adrift.
"I should have won this golf tournament by seven shots.Everybody knows that," said Garrigus on www.pgatour.com.
"If I could have just made a putt today. I don't know. It just wasn't starting on my lines. I hit a couple too firm and then I hit a couple too soft. I just couldn't get the feel for it."
Josh Teater and Bud Cauley had applied earlier pressure on the leaders as they carded 66s to close on 14 under alongside fellow American Chris Kirk.
Vijay Singh earned his third top-10 finish of the season after a closing 67, while English journeyman Gary Christian birdied the last two holes to cap a faultless 66 and earn a tie for 10th.