Horschel enjoyed a five-stroke lead at the start of a sun-soaked final round on Sunday in Ohio and no one ever got closer than two shots. He closed with an even-par 72 for 13-under.

Aaron Wise did what he could in a final round so difficult that no one shot better than 69. He and Chile's Joaquin Niemann were the only players to apply any serious pressure on Horschel.

Wise (71) opened the back nine with a pair of birdies and saved par on the 12th. But he dropped a shot on the 13th and the final hole to finish alone in second at nine-under.

Australia's Cameron Smith (77), who had the 36-hole lead, also started five shots behind. He had a pair of double bogeys for a 42 on the front nine and was never a factor, finishing three-under in a tie for 13th.

Horschel's streak of 49 consecutive holes without a bogey ended on the 6th hole. He didn't make his first birdie until the 10th and scrambled for bogey on the par-3 12th that dropped his lead to two over Wise.

"Having a five-shot lead, knowing it was mine to win, I really wanted to get the monkey off my back." - Billy Horschel.

Before the long eagle, Horschel saved himself with par putts of 12 feet on the 13th hole and 8 feet on 14.

And then it was over. From the front of the green on the par-5 15th, Horschel's putt from just inside 55 feet had the perfect line and perfect speed, bending left and dropping in the left side of the cup as he stretched out both arms in a quiet, disbelieving celebration.

"Just like you, big man," Horschel said to tournament host Jack Nicklaus when it was over.

Horschel said he has learned from Tiger Woods and Nicklaus that he didn't need to do anything special with a five-shot lead unless the moment called for it. It was calling on the 15th hole after Wise stuffed a wedge into two feet for birdie.

"If I had to do something special, I was ready for it ... Making that was huge," he said.

"Having a five-shot lead, knowing it was mine to win, I really wanted to get the monkey off my back."

Horschel, winner of the $2.16 million ($A2.99m) for the largest pay cheque of his career, now has seven PGA Tour victories and a three-year exemption.

He moves to just outside the top0-10 in the world, the highest he has ever been. He has three victories in the last 15 months, all of them against strong fields – the Match Play, the PGA Championship at Wentworth last year, and now this.