AKRON, Ohio — Justin Thomas took all the drama out of the final World Golf Championship at Firestone, never letting anyone closer than two shots and closing with a 1-under 69 to win the Bridgestone Invitational for his third PGA Tour title this season.

Sweeter than capturing his first World Golf Championship was the sight behind the 18th green Sunday.

His grandparents, Paul and Phyllis Thomas, watched him win for the first time on the PGA Tour. Paul Thomas was a career club professional and played at Firestone in the 1960 PGA Championship, missing the 54-hole cut. His son, Mike Thomas, also is a career club pro in Kentucky and a former PGA of America board member.

“I got a little choked up when I saw grandma and grandpa over there,” Thomas said. “It’s really cool. They don’t get to come out very often.”

They saw a one-man show.

Playing in the final group with Rory McIlroy, the 25-year-old Thomas made only two birdies. That was all he needed on a day when just about everyone within range was making all the mistakes.

McIlroy finished the back nine with consecutive bogeys and never recovered. Ian Poulter shot 74. Jason Day tried to make a run by making three straight birdies, only to play the final six holes in 5 over to shoot 73.

Tiger Woods, an eight-time winner at Firestone, started 11 shots behind and figured he would go out with a bang by playing aggressively. He turned in a dud, and a birdie on the 18th hole gave him another 73 to leave him 15 shots behind.

“Things could have certainly gone better,” Woods said. “But it is what it is, and on to next week.”

Thomas must feel the same way. He had gone five months since his last victory, a playoff win at the Honda Classic. While he didn’t feel as though he were playing poorly, he didn’t have the results to back it up. Now he does, and Thomas heads to St. Louis next week for the PGA Championship, where he will try to join Woods as the only players to win back-to-back in stroke play. Woods did it twice.

Thomas had not had a score better than 67, and he had not finished higher than a tie for 28th in his two previous appearances at Firestone.

“I’m glad I finally played well around here, just in time to leave,” he said.

Firestone has held tour events since the Rubber City Open in 1954. The World Series of Golf began in 1962, and it became an official PGA Tour event in 1976. In many respects, it was the precursor to the World Golf Championships by bringing in winners from around the world.

Bridgestone shifted its title sponsorship to the PGA Tour Champions, which will bring its Senior Players Championship to Firestone next year. The World Golf Championship instead will move to Memphis, Tennessee.

Thomas finished at 15-under 265 for a four-shot victory over Kyle Stanley, who got within two shots of the lead until bogeys on the 13th and 14th holes. Stanley closed with a 68.

Dustin Johnson, the world’s No. 1 player who was coming off a victory in the Canadian Open last week, started the final round 10 shots behind and shot 29 on the front nine. A birdie at No. 10 put him three shots behind, but that was all he had. Johnson bogeyed the last hole for a 64 and shared third with Thorbjorn Olesen of Denmark, who also had a 64.

U.S. Open champion Brooks Koepka had a 67 to finish fifth.

McIlroy won at Bay Hill in March and has three runner-up finishes, and he had said Saturday afternoon he was tired of finishing second. Not to worry. His 73 gave him a tie for sixth.

Thomas becomes the 21st player to win a World Golf Championship and a major, and his three victories tie him with Johnson and Bubba Watson for most on the PGA Tour this year. The ninth victory of his career moves him to No. 2 in the world, with a shot to regain the No. 1 ranking next week at the PGA Championship.

He set the tone early by hooking a pitching wedge over a steep lip in a fairway bunker to just short of the green and saving par with a 6-foot putt, then holing a 15-foot birdie putt on the par-5 second.

“It was big because I was really nervous today,” Thomas said. “It felt like it had been a while, but I guess it hadn’t really been that long. I don’t know. I was very nervous, very jittery. To make that putt on 1 and again on 2 just kind of calmed me and got me going for the day.”