Thomas becomes only the fourth player to win a major, The Players Championship, the FedExCup and a World Golf Championship – after Tiger Woods, Henrik Stenson and Rory McIlroy.

He carded a four-under 68 to outlast Lee Westwood, a hard-luck runner-up for the second straight week.

Westwood birdied the final hole for a 72 to finish one shot behind.

A shot further back at 12-under were Bryson DeChambeau (71), coming off a win last week at Bay Hill, and the unheralded Brian Harman (69).

RELATED: Thomas's winning bag

Thomas began the year with an anti-gay slur under his breath after missing a short putt.

That led to one corporate sponsor dropping him and another giving him a public reprimand.

The one time he had a chance to win, he learned his grandfather died before the final round of the Phoenix Open.

He was shaken until Sunday at the TPC Sawgrass, where he took on every shot and delivered a gem to finish at 14-under 274.

Thomas went birdie-birdie-eagle-birdie around the turn, and put away Westwood for good with a pair of lag putts from 50 feet – one for birdie on the par-5 16th to take the lead, another on the island-green 17th for a par.

"I fought so hard today. It's probably one of the best rounds of my life tee-to-green. I've seen some crazy stuff happen on TV in the past, and I'm glad to be on this side of it." – Justin Thomas

His 14th victory of his PGA Tour career will see him return to No.2 in the world.

"I fought so hard today," Thomas said.

"It's probably one of the best rounds of my life tee-to-green.

"I've seen some crazy stuff happen on TV in the past, and I'm glad to be on this side of it."

The 47-year-old Westwood's hopes of securing the biggest win of his long and successful career faded when he made bogey on the 17th to slip two shot behind Thomas.

Best of the Australians was Cameron Smith (72) in a tie for 17th at seven under, veteran Cameron Percy's debut at TPC Sawgrass concluded with a fine 68 to jump into a tie for 29th at four under, while Jason Day was a shot further back after a closing 73.

Adam Scott signed for a 71 to finish at one under, his record-setting play over the first three days on the 17th ending on Sunday when his tee shot found water, making double-bogey after three birdies on the famed island green this week, while Matt Jones (74) finished even.

Thomas was outside the cut line after nine holes on Friday.

He followed that with a 64 on Saturday to get in the mix, and he finished off in style to pick up US$2.7 million (AU$3.5m) with a win against the strongest and deepest field in golf.