Chris Kirk shot a bogey-free final round to clinch the Deutsche Bank Championship while Rory McIlroy finished joint-fifth at TPC Boston on Monday.
The American, who was joint third with Rory McIlroy and Jason Day on 10 under going into the final day, carded five birdies in a final-round 66 to beat Australia's Geoff Ogilvy and fellow Americans Russell Henley and Billy Horschel by two shots.
"Biggest win I ever had," Kirk said on pgatour.com shortly after sealing victory on 15 under overall. "I may be a little bit more calm, but I was still freaking out a bit inside."
Kirk's victory in the second of the four play-off events means the 29-year-old American now leads the FedEx Cup standings. It is only his third PGA Tour career title.
He wasted no time in eating into Henley's two-shot overnight lead by birdieing the third and fourth holes, while a further birdie on the ninth meant he had picked up three shots in the opening nine holes.
Kirk made par on the next three before birdieing again on the 13th and repeating the feat for the fifth and final time of his round on the 16th before finishing with two more pars.
Kirk's victory gives United States captain Tom Watson plenty to ponder ahead of the Ryder Cup at Gleneagles later this month.
Watson chooses his three wildcard picks on Tuesday and Kirk has done his chances of selection no harm at all.
"I definitely put myself in contention for one," Kirk said when asked about his Ryder Cup chances. "If I get picked, great. If not, I'm still happy with my win today."
World number one McIlroy, who had catapulted himself into contention with a flawless seven-under 64 on Sunday, could not secure his fourth title in five tournaments after recording four bogeys in his final-round 70 to finish joint fifth with Australia's John Senden on 11 under.
They were two shots shy of the second-placed trio of Ogilvy, Henley and Horschel.
Martin Kaymer and Day were tied for seventh on 10 under, while Sweden's Carl Pettersson and Jimmy Walker of America were two of seven players tied for ninth place on nine under.
World number two Adam Scott had to be content with a joint 16th-placed finish with six others on eight under, while England's Ian Poulter was a further shot back to finish alongside Jim Furyk and Rickie Fowler.