Colorado: Billy Horschel, seeking a second PGA Tour win, fired a sizzling 63 to surge three shots clear after a wildly fluctuating third round of multiple lead changes at the BMW Championship on Saturday.
As world number one Rory McIlroy finished nine strokes off the pace after carding a roller-coaster 72, American Horschel stormed ahead with five birdies after the turn to post a 13-under-par total of 197 at Cherry Hills Country Club.
Horschel, who clinched his maiden PGA Tour victory at the 2013 Zurich Classic of New Orleans, ended his round in style at the par-four 18th by sinking an uphill birdie from 32 feet which broke from right to left before dropping into the cup.
"I knew when I hit it that it had pretty good speed and was going to be close, and it fell right in over the top of the edge," Horschel told NBC Sport after taking control of the tournament with a bogey-free display.
"I`ve been seeing the (putting) lines pretty well this week and my speed`s been really good. It was a great day today. I played really solid, the goal today was making no bogeys and I played well."
American Ryan Palmer, who duelled for the lead with Horschel for much of a sunny afternoon at the revered venue just outside Denver, was alone in second place after returning a three-under 67 in the PGA Tour`s penultimate FedExCup playoff event.
U.S. Open champion Martin Kaymer of Germany shot an eight-birdie 64 to climb into a tie for third at eight under, ending the day level with Masters winner Bubba Watson of the United States, who carded a 66.
"I played really well today and had a lot chances and finally some putts dropped," said Kaymer, who by his own admission lost form due to fatigue for several weeks after his commanding eight-shot victory at the U.S. Open in June.
"Overall, today I shot six under par, and seven under the last 12 holes so that`s pretty good."
Four players - overnight pacesetter Sergio Garcia, Palmer, McIlroy and Horschel - held at least a share of the lead during the third round before Horschel took control.
The 27-year-old American, who tied for second at last week`s Deutsche Bank Championship after dumping his second shot on the final hole into a hazard, sank an eight-foot birdie putt at the 10th to move one stroke clear.
Briefly caught at the top by Palmer, Horschel again edged ahead when he drained a 22-footer at the 14th before tightening his grip on the tournament with further birdies at the 15th, 17th and the last.
Palmer, hunting his fourth career victory on the PGA Tour, mixed four birdies with two bogeys on his back nine and, like Horschel, finished his round in style by draining a 30-footer on the 18th green.
McIlroy, just two strokes of the pace heading into the third round, experienced an adventurous day as he carded four birdies, three bogeys and an ugly triple-bogey at the par-three 12th where he four-putted from four feet.
Garcia also slid back, struggling with his putting and recording just one birdie and three bogeys for a 72 and a six-under total of 204.
American world number 10 Phil Mickelson withdrew from the BMW Championship before the start of the third round, saying he wanted to rest for this month`s Ryder Cup in Scotland.
His compatriot Keegan Bradley also pulled out on Saturday, citing unease about a potential rules infraction over a drop taken at the par-four 18th in the opening round.
Bradley`s third shot on the hole had embedded in grass just above a bunker. After taking relief for a ball embedded in its own pitch mark, he ended up with a double-bogey six.
Though a PGA Tour rules official subsequently absolved Bradley of any violation, the American decided to withdrew from the event due to his own lingering doubts over the incident.
The top 30 players on the FedExCup points list after the BMW Championship will qualify for next week`s Tour Championship finale in Atlanta.