The World No.3 fended off brief challenges from Jason Day and Marc Leishman, as well as a valiant run from World No.1 Dustin Johnson, with a one-under-par 69 at Firestone Country Club.

At 15 under, Thomas finished four shots ahead of runner-up Kyle Stanley (68), with Johnson and Thorbjorn Olesen (both 64) sharing third at 10 under.

Thomas' ninth title on the PGA Tour will bank him a cool $US1.7 million cheque and will move him up one spot on the world rankings.

He said the victory was particularly special given his grandfather, who played in the 1960 US PGA Championship at Firestone, was in the gallery.

"I got a little choked up when I looked over and saw my grandpa and grandma over there," an emotional Thomas said.

"They don't get to come out that often anymore, so this is my first PGA Tour win with him here and it's really special."

RIGHT: Jason Day’s hopes faded with errant tee shots on the final few holes. PHOTO: Gregory Shamus/Getty Images.

Day and Leishman came within striking distance on the final day.

Former World No.1 Day pulled within two shots of Thomas when he rattled off three straight birdies starting at the 10th hole, but a series of errant tee shots and poor recoveries resulted in back-to-back bogeys from the 13th.

When Day chunked his wedge shot into the water at the par-5 16th and racked up a double-bogey, his title hopes were shot even before another dropped shot at the 17th.

Day signed for a 73 to sink to a share of 10th at seven under, while Leishman also carded 73 to finish a shot back in a tie for 14th.

Cameron Smith was next best of the Australians having signed off in style with a 67, improving his total to four under and outright 23rd place.

Smith's countryman Wade Ormsby played with eight-time WGC-Bridgestone winner Tiger Woods and both shot 73.

Woods finished at even par while Ormsby signed for a one-over total.

Adam Scott birdied the final hole but a 74 left the 2011 winner at four over.

Evin Priest, Australian Associated Press