Mexico City - Rory McIlroy started strong in his bid to complete his WGC trophy collection, firing a six-under par 65 on Thursday for a two-shot lead in the Mexico Championship.
The Northern Ireland star eagled his second hole of the day at Club de Golf Chapultepec, the par-five 11th.
He closed with back-to-back birdies to break free atop a crowded leaderboard with two-time Masters champion Bubba Watson and 2017 PGA Championship winner Justin Thomas two shots back on four-under 67.
It was a further stroke back to South African Louis Oosthuizen, Canadian Corey Conners and Americans Billy Horschel and Bryson DeChambeau.
"I took advantage of how I drove the ball," McIlroy said. "I made one bogey, I misjudged the lie on the fourth hole there, but apart from that just a really solid round of golf."
McIlroy, number one in the world again for the first time since 2015, is coming off a tie for fifth at the Genesis Open at Riviera in Los Angeles - where he shared the 54-hole lead but couldn't match eventual winner Adam Scott in the final round.
He said the Chapultepec greens were a little softer than those at Riviera, and a return to his old, 34-inch putter had him feeling more comfortable after he experimented with a 35-inch flat stick in California.
"Sometimes my right arm position could get a little high," he said, explaining the 35-inch putter let his right elbow "tuck in" better.
"But on right to left putts, it just felt as if the putter felt a little too long," he said.
With his "old faithful" back in the bag, McIlroy rolled in a 10-footer at the ninth to cap his round.
He'd drained a 27-footer at the sixth and a 15-footer at the eighth to move atop the leaderboard.
McIlroy won the WGC-HSBC Championship in Shanghai for the first time in November, and a victory on Sunday would see him complete the "grand slam" of the elite series which also includes the WGC Match Play - which he won in 2015 - and the WGC Invitational - which he won at Firestone in 2014.
The only player to have won all four is Dustin Johnson, who relegated McIlroy to second at Chapultepec last year.
"The first year we came here in '17, I think I had the lead after 36, didn't end up winning," McIlroy said. "Last year, finished second.
"I like the place, like the golf course. I've always played well at altitude. Hopefully this year I can go one better."
Watson said that with altitude affecting ball flight, the winds that kicked up in the afternoon were especially bothersome.
"The elevation already messes with your head and what you're thinking," he said. "And then you're talking about the wind, you're always second-guessing yourself."
Thomas, who like Watson had five birdies and one bogey in his round, echoed that sentiment.
"I've never played this place with this kind of wind," he said. "You can go so low, but, man, if you're not playing well you can shoot over par in a heartbeat.
"It's pretty tough to manage your score."
That was evident in the five-over round by defending champ Johnson.
Scott, whose Riviera triumph last week followed his Australian PGA Championship win in December, was three-over after 18 holes.
Leading first-round scores on Thursday in the World Golf Championships Mexico Championship at Club de Golf Chapultepec (USA unless noted, par-71):
65 - Rory McIlroy (NIR)
67 - Bubba Watson, Justin Thomas
68 - Louis Oosthuizen (RSA), Billy Horschel, Corey Conners (CAN), Bryson DeChambeau
69 - Tyrrell Hatton (ENG), Paul Casey (ENG), Patrick Reed, Im Sung-jae (KOR), Lee Westwood (ENG), Hideki Matsuyama (JPN)
70 - Abraham Ancer (MEX), Tommy Fleetwood (ENG), Gary Woodland, Bernd Wiesberger (AUT), Erik van Rooyen (RSA)
71 - Lucas Glover, Rafa Cabrera Bello (ESP), Pablo Larrazabal (ESP), Branden Grace (RSA), Chez Reavie, Zach Murray (AUS), Justin Harding (RSA), Sebastian Munoz (COL), Kevin Na, Matthias Schwab (AUT)