It was every bit of that.
With five players still in the mix in the final hour, Thomas surged ahead by chipping in from 40 feet on the par-3 13th hole, and holding his nerve down the stretch as his challenger eventually faded, one after another.
Hideki Matsuyama, bidding to become the first player from Japan to win a major, appeared to recover from back-to-back bogeys with two straight birdies on the 14th and 15th holes to get within one shot. But the championship turned on the 16th hole.
Thomas faced a 6-foot par putt to stay at 8 under. Matsuyama caught a good lie over the green and chipped to 5 feet. Thomas wasted no time over the putt and drilled it in the center of cup. Matsuyama missed and was two shots behind.
Thomas sealed it with a 7-iron from 221 yards that cleared the water and rolled out to 15 feet. The birdie putt curled in and his lead was up to three going to the 18th. A final bogey only affected the score.
He finished at 8-under 276, his fourth victory of the year.
Kevin Kisner was the last one who had a chance to catch him. But he three-putted from 100 feet on the 16th for bogey, couldn't birdie the 17th from long range and hit his second shot into the water and finished with a double bogey. Kisner, the 54-hole leader, closed with a 74.
Matsuyama also hit into the water on No. 18 and made bogey for a 72 to finish three back.
Louis Oosthuizen (70), Patrick Reed (67) and Francesco Molinari (67) tied for second, though none had a chance to win playing the 18th.
Kiwi Ryan Fox finished off the pace in 54th following a final round seven-onder 78.