Nick Watney shot a final-round 66 to win the AT&T National in Pennsylvania, beating KJ Choi by two strokes.
The pair were level going into the final stages but the Korean posted a costly double bogey on the 15th.
American Watney birdied the 16th and parred the final two holes to clinch victory.
Joint overnight leader Rickie Fowler double bogeyed the second and carded a 74 to finish in 13th.
Australia's Robert Allenby almost put himself in contention when he notched a hole-in-one on the par three 14th, but a bogey on the next hole hurt his momentum.
The 39-year-old eventually finished joint sixth with Chris Stroud.
Englishman Justin Rose was the nearest European contender in the pack, finishing in joint 15th place at four under par.
Choi admitted his errors on 15 had been key.
He said: "When I tied him on the 14th hole, I knew that there was still a lot of holes to go, and I knew the remaining holes were more favourable to Nick Watney, and I knew the 15th hole would be a turning point.
"That was a key hole, and I missed it. So I think that was the turning point of the match.
"The tee shot is a very difficult tee shot. It's a very long distance. I tended to hit it on the right side on that hole this week, so when you hit it on the right side, the rough is very difficult, so you really don't have a chance if you miss the fairway there."