Mexico City - Justin Thomas fired a six-under par 65 to seize a one-stroke lead over fellow American Patrick Reed and South Africa's Erik van Rooyen after Saturday's third round of the WGC Mexico Championship.
World number four Thomas, the 2017 PGA Championship winner, opened and closed with bogeys but made eight birdies in between to stand on 15-under 198 after 54 holes at Chapultepec Golf Club.
"I would have liked to have birdied the first hole to have seen what would have happened - I hit four bad shots on that hole," Thomas said.
"I've gotten better just in general. I feel like I'm playing well enough where if I don't play them all well I can play four or five in a row and get pretty hot."
Thomas has managed three top-10 finishes at the Mexico event, including a runner-up effort to Phil Mickelson in 2018, but he yearns to finally capture the trophy.
"I really want that first place," Thomas said. "I've been close a couple times. I definitely feel like I have a little bit of unfinished business here.
"I know I have to play well tomorrow but I want it."
Reed, the 2018 Masters champion, closed with a bogey to settle for a share of second with van Rooyen after both shot 67 to finish on 199.
Spain's Jon Rahm shot a course-record 10-under 61, with a one-bounce ace at the par-3 17th, to share fourth on 202 with world number one Rory McIlroy of Northern Ireland and American Bryson DeChambeau.
England's Paul Casey and Tyrrell Hatton shared seventh on 203.
Thomas opened with a bogey, going well right of the cart path off the tee, but answered with birdies on the next two holes, sinking a 16-foot putt at the par-3 third, and made another from seven feet at the par-5 sixth.
He began the back nine with back-to-back birdies, made two more at the 14th and par-5 15th then dropped his approach just inside 14 feet at the par-3 17th and made the birdie putt.
Another bogey at 18, where Thomas missed a par putt from six feet, opened the door for Reed, but he couldn't charge through.
Reed had back-to-back birdies to start the back nine and again at 14 and 15 but he found the right rough off the 18th tee and a greenside bunker, then blasted out to the fringe and came up short on a 17-foot par putt.
Rahm birdied six of the first seven holes before making a three-putt bogey from 45 feet at the par-4 eighth. He birdied the par-4 10th, par-3 13th and 15 before holing out from the 17th tee.
"I think a lot of us like to talk sometimes about how unlucky we get -- today I got fortunate," Rahm said. "I hit the right shots and I got the right bounces and I took advantage of it."
He said he knew it could be a special day when he had four birdies to open his round.
"When I basically had tap-in, tap-in, tap-in, and 10 feet for a birdie," said Rahm, who said his goal was "to get as close as possible to the leaders."
198 - Justin Thomas 67-66-65
199 - Patrick Reed 69-63-67, Erik van Rooyen (RSA) 70-62-67
202 - Jon Rahm (ESP) 72-69-61, Rory McIlroy (NIR) 65-69-68, Bryson DeChambeau 68-63-71
203 - Paul Casey (ENG) 69-68-66, Tyrrell Hatton (ENG) 69-68-66
204 - Gary Woodland 70-69-65, Kevin Na 71-68-65, Hideki Matsuyama (JPN) 69-64-71