TULSA, Okla. — The dominant story line before the 2022 P.G.A. Championship revolved around Phil Mickelson, who became the oldest major champion last year when he won the event at age 50 but chose not to defend his title this year. Then the focus of the tournament shifted to Tiger Woods, 46, who came to the Southern Hills Country Club to resume his stirring comeback from injuries sustained in a horrific car crash 15 months ago. But Woods struggled physically, and mired in last place after three rounds, he withdrew before Sunday’s final round.

What evolved instead on the last day of this year’s P.G.A. Championship was yet another dramatic showdown between the dazzling, hard-swinging 20-somethings overtaking elite golf.

In a three-hole aggregate playoff after Sunday’s 18-hole fourth round ended in a tie, Justin Thomas, 29, held off the rising star Will Zalatoris, 25, to win his second P.G.A. Championship. Thomas, who entered Sunday’s fourth round seven strokes out of the lead, also won the 2017 P.G.A. Championship.

Thomas and Zalatoris began the playoff with birdies on the first playoff hole, the 13th. On the reachable par-4, 302-yard 17th hole, Thomas drove the green and had a lengthy putt for eagle that came up 3 feet short. Zalatoris’s drive on the 17th was just off the green and his flop shot stopped 8 feet from the hole. His birdie putt skidded past the hole, and Zalatoris tapped in for par.

With the chance to seize the advantage, Thomas rattled in his birdie putt for a one-stroke edge heading into the third playoff hole, which was the 18th.

Both golfers reached the 18th green in two. Zalatoris could not covert a birdie putt and Thomas needed only two putts for a par that clinched the championship.

It was the first playoff in a major championship since the 2017 Masters.

At various times in the latter stages of the fourth round, both Thomas and Zalatoris appeared incapable of catching Mito Pereira, who was the third-round leader and seemed poised to become the first golfer from Chile to win a major championship.

Pereira, playing in just his second major championship, held a one-stroke lead entering the 18th hole. But he sliced his tee shot on the final hole into a small creek adjacent to the fairway. After a penalty shot drop from the water, Pereira’s approach shot found the thick rough alongside the green. His chip from there trundled far across the green until it stopped in the fringe on the opposite side of the green. Pereira, 27, made double bogey, which dropped him into a tie for third place with the American Cameron Young.

Zalatoris has a wealth of major championship experience despite being on the PGA Tour for just three seasons. He finished second at last year’s Masters and was tied for sixth at that event last month. He was tied for eighth at the 2021 P.G.A. Championship and tied for sixth at the 2020 U.S. Open.

But on Sunday, Zalatoris, after an even-par front nine, was hampered by poor putting, something that has plagued him all season. He bogeyed the 12th and 16th holes, but rallied when he took an aggressive line off the tee at the reachable par-4 17th hole. His ball tumbled into a greenside bunker, but he escaped with a deft sand shot that stopped 8 feet from the pin. Zalatoris steadied his nerves and sank the putt for birdie. He closed by making a 10-foot putt to save par at the final hole to shoot 71 and finish at five under par overall. At the time, though, it did not appear to be enough to catch Pereira.

Justin Thomas most likely finished his round with the same feeling. He started Sunday seven strokes behind Pereira, but made a dramatic surge after playing the first eight holes in one over par. After a birdie at the ninth hole and a par at the 10th, Thomas’s tee shot at the par-3 11th hole was slightly short of the green.

Thomas then sank the birdie putt from 64 feet. At the par-4 12th, he sank an 18-foot birdie putt for birdie. Thomas missed consecutive makable birdie putts at the 13th and 14th, but then splashed a shot from a greenside bunker at the par-4 17th hole to within 3 feet, a distance he successfully negotiated for his fifth birdie of the day. That would put him within one stroke of Pereira with one hole to play. A brilliant drive and a courageous approach shot to the elevated 18th green stopped 11 feet behind the hole, but Thomas’s putt slid past the right edge for a par and a score of 67.