There was no need to try anything special on a course playing as tough as Muirfield Village.
And then the situation called for something special, and Horschel delivered.
With his lead down to two shots, Horschel produced a signature moment of his own Sunday by making an eagle putt from one end of the green to the other on the 15th hole, sending him to a four-shot victory over Aaron Wise.
"If I had to do something special, I was ready for it," Horschel said. "Making that was huge."
No one ever got closer than two shots on a sun-soaked final round. Horschel closed with a 72. There were still a few nervous moments.
Horschel's streak of 49 consecutive holes without a bogey ended on the sixth hole. He didn't make his first birdie until the 10th hole. He had to scramble for bogey on the par-3 12th that dropped his lead to two over Wise.
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 Nicklaus when it was over.
His lead was up to four shots, and it was a comfortable finish. Horschel finished at 13-under 275 and won $2.16 million, the largest paycheck of his career. As an elevated event, the win comes with a three-year exemption.
Nicklaus as impressed with Horschel's two par saves on the back nine as his memorable eagle.
"I look at putting as you make putts when you have to make putts," said Nicklaus, among the best at that in golf history. "What counts is you make them when you made them. That to me that's the mark of a champion. And those are the guys who win tournaments, and you did that and you won."
Wise did what he could in a final round so difficult that no one shot better than 69. He and Joaquin Niemann were the only players to apply any serious pressure on Horschel. He opened the back nine with a pair of birdies sand saved par from the back bunker on the 12th. But he dropped a shot on the 13th just as Horschel was looking shaky.
His wedge to the 15th spun back to tap-in range to the hole, and Wise thought he had a chance with Horschel facing such a long putt.
"I thought that was going to be a big turning point for me," Wise said. "Instead it was kind of the opposite for me. It was a birdie for me, and Billy holed a 50-footer for eagle. That was the turning point for him."
Wise made a meaningless bogey on the final hole for a 71 to finish alone in second. The consolation prize was a day off on Monday. Wise moved from No. 88 to No. 44 in the world ranking and is now exempt from 36-hole U.S. Open qualifying.
Luke List moved to No. 59 and also qualified for the U.S. Open.
Cameron Smith, 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.
Niemann, who won another elevated event at Riviera in the Genesis Invitational, made a strong move and was creeping within range until his wedge on the 14th found a bunker, leading to double bogey. He answered with two birdies, finished with a double bogey and shot 71. He tied for third with defending champion Patrick Cantlay (71).
Before the handshake with Nicklaus, Horschel was mobbed by his three children. He now has seven PGA Tour victories. His parents had seen him win. But it was the first time his wife and three children were there, and the kids were bouncing on the firm greens.
That might have been as great as any pressure as Horschel felt.
"Having a five-shot lead, knowing it was mine to win, I really wanted to get the monkey off my back," he said of winning with his family in attendance.
Horschel moves to No. 11 in the world, the highest he has ever been, thanks to a year that finally has brought some consistency in a hit-and-miss game. He has three victories in the last 15 months, all of them against strong fields - the Dell Match Play and the BMW PGA Championship at Wentworth last year, and now this.
It might even be enough to finally be considered for a U.S. team with the Presidents Cup later this year.