"I never hit it out of position and think I got away with one bad tee shot where it bounced out of the trees but apart from that played lovely, putted really nice and just picked them off. So it's pretty cool".
Northern Irishman Rory McIlroy battled inconsistent wedge play, his frustration eased by a holed bunker shot at the 17th, which helped him join South Korean Kim Si-woo three back.
The concept of whacking a tiny ball with a club - you know how golf works - and making it go where you want is ridiculous on its own. He would make just one par on the back nine to go along with three birdies, three bogeys and two double-bogeys for a four-over 40.
Tiger Woods, an eight-time victor at Firestone, started 11 shots behind and figured he would go out with a bang by playing aggressively.
Grandfather Paul played in the 1960 PGA Championship at Firestone. Tiger Woods is finishing up on the course.
"Things could have certainly gone better but it is what it is and (we're) on to next week".
Thomas finished on 15 under after a one-under 69 on the final day.
And a birdie from 12 feet on the 13th effectively sealed a first World Golf Championship title and the winner's cheque of $1.3m for Thomas ahead of the final major of the season at Bellerive Country Club.More news: Ryanair release strong statement blaming "handful" of Aer Lingus pilots for strikes
Now, three victories later, he has closed the gap on Spieth - 11 PGA Tour victories for Spieth, nine for Thomas.
AKRON, Ohio - The last time Justin Thomas won a golf tournament, he was calling out rowdy spectators in the gallery and getting more attention for that controversial move than for capturing the Honda Classic.
What made Sunday even better for Thomas was having his grandparents on hand.
But McIlroy had already dropped a shot at the short fifth and effectively took himself out of contention with a torrid run of three consecutive bogeys around the turn, while Poulter's chances were scuppered when he dropped three shots in four holes in an outward 37. Maybe I might be here when the Senior Tour comes around. The World Golf Championship instead will move to Memphis, Tennessee.
"He wasn't hitting it as far as he normally does, " said Marc Leishman, who played with Woods and beat him by six shots.
The 29-year-old mixed five birdies with two bogeys in a round that he admitted frustrated him as he struggled off the tee on the tight tree-lined fairways.
Fellow American Kyle Stanley finished second on 11 under, with Denmark's Thorbjorn Olesen and American Dustin Johnson a further shot behind in tied third.
McIlroy remained the most likely threat early on when he matched Thomas' birdie at the second, and Thomas then opened the door for the chasing pack when a poor drive led to a bogey at the sixth.