NORTON, Mass. (AP) -- Justin Thomas added another big title to a season filled with big moments by outlasting Jordan Spieth and Marc Leishman on the back nine of the TPC Boston to win the Dell Technologies Championship.
Two shots behind at the turn, Thomas surged ahead with a short birdie and a 6-foot par save while limiting the mistakes that cost his challengers. He closed with a 5-under 66 for a three-shot victory that was much tighter than the margin suggested.
Spieth wasted a start that riled up the Labor Day crowd - birdie-eagle-birdie-birdie - with too many mistakes on the back nine. He fell out of a share of the lead by missing the 14th green with a 5-iron from the fairway. Needing an eagle on the par-5 18th to stay in the game, he pushed a 4-iron into a tough lie in the bunker, blasted over the green and closed with a bogey for a 67.
Spieth was runner-up for the second straight week in the FedEx Cup playoffs, though that was enough to move up to No. 1 in the standings with one more event before the $10 million prize is decided at the Tour Championship.
Leishman shot 30 on the front to build a two-shot lead, only for it to vanish quickly. He had to save bogey on No. 10 after an errant drive. He three-putted from long range for bogey on the 11th. He found a bunker on the 12th for a third straight bogey. And then he closed with two bogeys that only cost him money. Leishman shot 70.
For the 24-year-old Thomas, his breakthrough season just keeps getting better.
He won for the fifth time this season, including the PGA Championship for his first major. This one might make him a shoo-in for PGA Tour player of the year unless Spieth, the British Open champion, were to win the final two events for the FedEx Cup title.
"We've just had a great year," Thomas said. "Obviously, there are two more events to finish it off."
Thomas moves to No. 4 in the world.
Top-ranked Dustin Johnson, coming off his fourth victory of the season last week on Long Island, started the final round three shots behind and was one of the few players who was never in contention. He took two to get out of a fairway bunker and made bogey on the par-5 second hole, and he closed with a 73 to finish 10 behind.
Spieth was simply dynamic at the start, with no putt longer than 12 feet in going 5 under for the opening four holes to briefly take the lead. Leishman, who won at Bay Hill in March, kept stride with six birdies on the front nine to match Spieth's 30 and take the lead.
Thomas did his part to stay in the game.
He could hear the crowd celebrating Spieth's short birdie on the fourth hole, and then Thomas belted a low slider of a drive that barely cleared the deep bunker, hit into the shaggy collar and hopped onto the fringe. He holed that 30-footer for an eagle, and the race was on.
Phil Mickelson birdied three of his opening four holes. He couldn't keep pace. Neither could Jon Rahm or Paul Casey.
The final round was so tight that Thomas, Spieth and Leishman were tied for the lead with seven holes to play. But then it all changed when Spieth missed the green at No. 14, did well to hit a flop shot to 15 feet and narrowly missed his par putt.
Thomas steadied himself after his lone bogey on the par-3 11th. Equipped with a one-shot lead because of Spieth's bogey, Thomas hit a sand wedge to 4 feet on the 15th for a birdie and a two-shot lead. Equally important was his pitch out of deep rough to the right of the 16th to a green that ran away from him. He managed to hit it 6 feet and twice clutched his fist when it dropped for par.
Spieth grazed the edge of the cup with a 10-foot birdie attempt on the 17th, and then lost all hope with his approach into the bunker.
It was the first time Spieth has finished runner-up to Thomas, his best friend in golf since they were teenagers.
Kevin Chappell bogeyed his last hole and still managed to nudge Charley Hoffman by a fraction of a point to earn the 10th and final automatic spot on the U.S. team for the Presidents Cup.
Stewart Cink closed with a 68 to finish 12th, more than enough for him to advance to the third FedEx Cup playoff event outside Chicago for the first time in seven years. Emiliano Grillo and Rafa Cabrera Bello also moved into the top 70 to advance to the BMW Championship in two weeks.