At least Jason Day's bid lingered into the back nine, rattling off three consecutive birdies starting at the 10th hole to move to within two strokes of Thomas before a bogey at No. 13 set the stage for an uninspired finish.
This is only the fourth time he has been five shots or closer going into the weekend this year, but there are plenty of others around him. 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, tried to go out with a bang and ended with a dud.
The impromptu meeting was much more than simply a pep talk, the reigning PGA Tour player of the year doesn't need that.
After shooting three-over par in Round 3, Tiger Woods says he was fighting "everything" and that he didn't warm up well or hit the ball crisply.
"We're obviously doing a lot of things right".
Thomas could not have asked for a better week.
It is Sunday at the WGC-Bridgestone Invitational. The round included just one bogey, two birdies and 15 pars, but ended emotionally for J.T.
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Northeast Ohio has been the place for thousands to watch world class golf for years, however, Sunday was the last time Firestone Country Club's famed South Course will host the prestigious Bridgestone Invitational. We underrate how hard it is to present sufficient encores to elite seasons (which is what past year was), but Thomas has made it look fairly simple with maybe the most complete game on tour.
"To win on a championship golf course like [Firestone] and against a very tough field, it just felt great", he said.
His grandparents were both in attendance, and Thomas clearly wanted to win with them there.
He would make the turn in one-under 34, but his bogey on 10 would serve as a harbinger of uneven play to come.
As for Firestone, it has been an incredible run for Woods at the course that will no longer be host for the tournament - it is moving to Memphis next year.
He also finished third and fifth in his two appearances in the old World Series of Golf that was played at Firestone prior to the start of the World Golf Championship events. I loved playing here, even when it was the World Series of Golf.
"My grandpa is not emotional, but I hadn't seen him smile or be that happy, maybe ever". Also five shots back were Jon Rahm of Spain, who had a sloppy finish for a 70, Tony Finau (68) and Players champion Webb Simpson (65). It is moving to Memphis, Tennessee, in 2019.
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. He is paired alongside Rory McIlroy and Justin Thomas for the first two rounds.