The world number two American hit five birdies in the middle of his round on the way to a three-under-par 67.

This puts him on a score of 14-under for the tournament as he seeks a third win of the year at the Firestone Country Club.

"The hard part about this course is that you can get off track pretty quick if you're not playing well," said Thomas.

"That's why with it being a little softer this week you've seen a big variety of scores, because it's soft enough where if you're playing well you can shoot low, but if you're not, you can shoot some pretty high numbers."

Four-time major winner Rory McIlroy of Northern Ireland and England's Ian Poulter are tied for second place on 11-under.

McIlroy also carded a third round of 67 while Poulter, who led on his own after round one and was tied for the lead after round two, shot 70.

Australia's Jason Day remains in contention a further shot back in a top of the leaderboard with a distinctly international flavour.

Fourteen-time major champion Tiger Woods could only manage a 73 to slip back to three under, with the American 11 off the lead. 

The tournament will be followed by the Professional Golf Association Championships, the final major of the season.

Thomas, McIlroy and Woods will play in a group together for the first two rounds.

Thomas will start as defending champion.