Horschel ran off four straight birdies on the front nine to get in the mix, began to pull away with an eagle on the par-5 12th hole and closed with a nine-under 63 for a two-shot victory in the Corales Puntacana Championship on Sunday.
"This game of golf is so fickle," Horschel said. "You can put a lot into it and not get everything you want out of it. I knew ... I had the ability, I had the talent. I had to continue to believe the good stuff was going to come to the forefront."
Horschel, who started the final round three shots behind, all but sealed it with an 18-foot birdie putt on the 16th hole, finishing at 23-under 265 and earned $US720,000..
Wesley Bryan, trying to go wire-to-wire, was still only one shot behind late in the round. But right after Horschel made his final birdie, Bryan missed a par par putt from just outside two feet and fell three behind.
Bryan shot a 68 and was runner-up, which will get him into the AT&T Byron Nelson in two weeks. He was playing on a sponsor exemption. Kevin Tway shot 69 and finished third.
Australian Harrison Endycott finished tied 33rd, 13 shots behind Horschel after a closing 69.
Horschel won for the eighth time on the PGA Tour and the first time in nearly two years.
This gets him into the PGA Championship next month, and he likely will be in the next $US20 million signature event at the Wells Fargo Championship.
Those are the tournaments Horschel, a former FedEx Cup champion, was used to playing.
The opposite-field events are seen as a chance for younger players to get their first win. In the case of Horschel, it was crucial to get some confidence back.
He delivered one of his best closing rounds when he needed it — the four straight birdies starting on the par-3 second hole, a 31 on the front nine, the eagle on No. 12 to seize control and no bogeys over the last 30 holes of the tournament.
It was nearly a year ago at the Memorial that Horschel, the defending champion at Muirfield Village, shot 84 and was near tears talking about how low his confidence had fallen. He missed the FedEx Cup playoffs for the first time since 2012.
He had shown signs this year with a pair of top 10s, and now has a victory for validation. Along with getting into the PGA Championship at Valhalla, he is assured of starting next year at Kapalua in The Sentry.
18°C