He shot 73 to finish 15 shots behind.
And then he almost lost it.
Day and world No.19 Leishman have both revealed Australian former US PGA Tour player Jarrod Lyle's decision to end cancer treatment and enter palliative care as the inspiration to try and win at the elite, 71-man event.
Off the tee Thomas uses Titleist's new TS3 driver, a club he put in play the first week it was on the USGA's Conforming List of Driver Heads at the U.S. Open at Shinnecock. His grandmother is one of his biggest supporters who navigated her way around the hills of Firestone using a walker in 90-degree heat.
That made what will likely be his last-ever competitive round at Firestone a virtual practice round for next week's PGA Championship.
"I didn't hit the ball as clean as I wanted to, " he said. Of the four majors this year, though, this one will probably be the least-discussed when it comes to the course, which means the focus will be on the players. I hit some better wedge shots out there, which was good to see.
"I got a little choked up when I saw grandma and grandpa over there", Thomas said. In his last nine stroke play events on the PGA Tour, he's had a win and six top 15s.
Tommy Fleetwood, who had shared the second-round lead with Poulter, shot 74 to fall back to seven under.
Tiger Woods was never in the picture.
"The first 10 holes were pretty good", said Johnson. "But it is what it is, and on to next week".
All he needs to do is ask his good buddy, Jordan Spieth, what it's been like this year. He's looking to become the second three-time victor of the event. He lost in a playoff to Phil Mickelson the next week at the WGC-Mexico Championship and was fourth at the WGC-Dell Technologies Match Play. He lost in the semifinals of the Match Play.
Sunday at Firestone will be the eighth time this year they play together in a tournament, and the number grows next week when they play the opening two rounds with Tiger Woods at the PGA Championship.
Major Champion Jason Day was another shot back on ten under after his third-round 69, which included a chip-in birdie at the short 15th.
"It's a great opportunity to test what I've been trying to work on this week, test it under the gun on a Sunday when you have a chance to win".
Adding to the emotion of 25-year-old Thomas' victory was the joy of having his grandparents, Paul and Phyllis Thomas, in the gallery.
So ended a finale at Firestone that otherwise lacked drama. He was even for the week and in a tie for 32nd when he finished.