American Justin Thomas aced the 232-yard 13th hole on his way to firing a five-under par 66, grabbing a one-stroke lead at the World Golf Championships Mexico Championship.
Thomas, seeking his fifth PGA title and third triumph of the year, stood on 12-under 201 after 54 holes with reigning US Open champion and world number one Dustin Johnson one stroke off the pace after matching Thomas with his second consecutive 66.
Thomas made five birdies and two bogeys but his shot of the day was the hole-in-one at 13 where his tee shot bounced high on the front of the green and dropped into the bottom of the cup.
"That looked nice," Thomas said. "It was really cool. I hadn't played too many shots exactly how I wanted today. That was the first one and I liked that better for sure."
Five-time major champion Phil Mickelson and four-time major winner Rory McIlroy shared third on 203 with England's Lee Westwood and Spain's Jon Rahm another shot adrift.
Northern Ireland's McIlroy could overtake Johnson for world number one with a victory if the American finishes in a share of fourth or worse.
"I feel really confident with the golf ball right now. I've just got to go out and see what I can do tomorrow," Johnson said. "I played really solid. I'm hitting the ball really good and rolling it good too."
Thomas won the 2015 and 2016 CIMB Classic titles in Kuala Lumpur, then followed with back-to-back crowns in the first two PGA events of 2017, the Tournament of Champions and Sony Open, both in Hawaii.
"I'm very comfortable," Thomas said of leading with major winners hot on his heels. "The main thing is getting comfortable with my game... I had some great saves for par that saved my round."
Thomas birdied the second hole but found water off the seventh tee took a bogey at the par-3 hole, then rescued par at nine after finding the trees off the tee.
A 21-foot birdie at 10 jumpstarted his back nine and Thomas put his approach four feet from the cup to set up a birdie at 12, setting the stage for the roar-inducing ace at 13.
After a bogey at 14, Thomas responded with birdies at the par-5 15th, chipping from the rough to four feet, and par-4 16th, sinking an eight-foot putt.
- DJ's ball drops too late -
Johnson opened with a 20-foot eagle putt and sank a 17-foot birdie putt at the second, then dropped his approach inches from the cup for a tap-in birdie at the par-5 sixth.
A 15-foot birdie at the par-3 13th and six-foot birdie putt at the par-5 15th put Johnson atop the leaderboard but he stumbled at 16, taking a penalty stroke when his ball became stuck in a tree, only to have the ball fall to the ground moments after he had taken a penalty stroke for a lost ball and played on.
"I was just laughing," Johnson said. "If it's not the greens it's something else going against me right now."
- Spieth, Mickelson sizzle -
McIlroy fired a 70 to stay in the title hunt.
Mickelson endured a wild back nine that featured a fan picking up his ball after a shot and twice being given relief from sprinkler heads within bushes that disrupted his intended stance. He ran off three birdies in a row starting at 15 but a closing bogey left him on 68.
Two-time major champion Jordan Spieth fired a course-record 63 to share 10th on 206 and approach the final round with renewed confidence after a bogey-free day.
"Hit it a little straighter and was more aggressive," Spieth said. "I hit it close to the hole most of the day and didn't have to work too hard with the putter to get to 63 and get back in it. If I can do it today I can do it tomorrow."