Jon Rahm beat Dustin Johnson on the first hole of a play-off in a dramatic climax to the BMW Championship at Olympia Fields in Illinois, Chicago.
Spaniard Rahm, 25, looked set to be crowned champion after a bogey-free six-under 64 left him on four under.
But Johnson, who won last week by 11 shots, birdied the 15th and holed a 43-foot putt on 18 to force a play-off.
Rahm then holed an even longer putt on the same green in the play-off while the American’s effort finished short.
“I still can’t believe what just happened the last hour of play,” Rahm said.
“That stretch of waiting for DJ, him making the putt, going in the play-off, me making the putt, then trying to stay mentally in it just in case he made the last putt. It’s been a rollercoaster, but so much fun.”
Rahm came from three shots behind going into the day after Johnson was tied for the lead with Japan’s Hideki Matsuyama heading into Sunday’s round.
The Ryder Cup player made back-to-back birdies at 15 and 16 in a sparkling round to set the clubhouse target.
His only dropped shot of the weekend was a one-stroke penalty for picking up his ball without having marked it on Saturday.
World number one Johnson admitted he did not expect his long-range putt on the 18th to go in.
“I was kind of laughing at my putt, because it was one of those where you’re not really thinking you’re going to make it,” he said.
“I played an unbelievable putt, got in the play-off and then Jon made an even more ridiculous putt on top of me.”
Chile’s Joaquin Niemann and Matsuyama finished a further stroke back at two under on the North course at Olympia Fields.
England’s Matthew Fitzpatrick ended four shots off the winning score at level par after a final-day 67, while Northern Ireland’s Rory McIlroy finished on three over after a 71.
England’s Paul Casey (68) and Tyrrell Hatton (70) were a shot further back on four over.
The top 30 players in the points table have qualified for the season-ending Tour Championship at East Lake in Atlanta, starting on Thursday.
Tiger Woods, who needed a top-five finish, is among those to miss out.