Thomas matched a PGA Championship record when he rallied from a seven-shot deficit at Southern Hills, and then saved his most exquisite shot-making for a three-hole play-off to defeat Will Zalatoris.

He closed with a three-under 67, matching the low score of a final round made difficult more by nerves than the wind.

Thomas seized control in the play-off with a 3-wood to 35 feet on the 301-yard 17th hole for a two-putt birdie.

He tapped in for par and stood erect with a smile, a mixture of joy and disbelief.

"I was asked early in the week what lead is safe and I said, 'no lead,'" Thomas said. "I can't believe I found myself in a play-off."

Thomas needed plenty of help, and Mito Pereira provided it in a tragic finish.

The 27-year-old from Chile, playing in only his second major, took a one-shot lead to the final hole and drove into a creek to make double bogey.

Zalatoris looked like he had thrown away his chances for a first major - and first PGA Tour victory - when he three-putted from just outside 20 feet on the 16th hole.

But he responded with a birdie from the bunker at the 17th and holed an 8-foot par putt on the 18th for a 71.

He joined Thomas at 5-under 275, and they played on when Pereira faltered.

Thomas, who had gone 14 months since his last victory at The Players Championship last year, now has a PGA Tour victory in each of his past eight years and moves to No. 5 in the world.

John Mahaffey in the 1978 PGA Championship at Oakmont was the other player to come from seven shots behind on the final day.

He also won in a play-off over Tom Watson and Jerry Pate.

Thomas was still seven shots behind when he made his remarkable run, a mixture of key birdies and keeping mistakes off his card.

It started with a 65-foot birdie putt from just short of the green to a back pin on the par-3 11th.

He was lurking, while the leading pack behind him was leaking oil.

Pereira was on the cusp of becoming Chile's first major champion, and giving South America the career grand slam.

Even after five bogeys, he never lost the lead and delivered clutch par saves from the bunker left of the ninth green and from well behind the 10th green.

None was bigger than his 12-foot putt on the 16th to stay one shot ahead.

It all came undone with one swing.

His sawed-off swing with the driver peeled to the right and into the creek down the right side of the 18th fairway.

His double bogey gave him a 75, a hard-luck end to such a promising week.

"On Monday, I just wanted to make the cut. On Sunday, I wanted to win," Pereira said.

"I'll take this to learn for the future."

Cameron Smith (69) and Lucas Herbert (71) finished as the leading Australians in a tie for 13th at even par for the championship.

Australian Associated Press