The four-time major champion posted seven birdies and an eagle after starting his round on the 10th hole at the Chapultepec Golf Club in Naucalpan.

The United States' Dustin Johnson, the champion in 2017, began his round with three consecutive birdies and went on to fire a bogey-free 64 to sit in second place.

McIlroy, who won the most recent of his 14 Professional Golfers' Association Tour titles at the Arnold Palmer Invitational in March 2018, eagled his 10th hole.

The 29-year-old's only dropped shot came at his 15th, where he pulled his drive into the trees before an initial effort to get his ball back into play was scuppered by a branch.

He did well to limit the damage to a bogey six and pulled the dropped shot back with a superb putt from 20 feet at his 17th before a par at the last capped off an excellent round.

"It felt pretty stress-free but I wouldn't say it was easy," McIlroy, the world number eight, was reported as saying by BBC Sport.

"I hit a lot of good shots, I left myself a lot of tap-ins for birdies and had a nice eagle on the first hole as well."

He added: "If I'm excited about anything in my game, it's the way I've putted the last few weeks.

"Also my attitude - I'm not putting myself under any pressure, I've got things in a really nice perspective. 

"I think that's probably 80 per cent of the reason why I'm playing good golf over the first few weeks of the year."

Defending champion Phil Mickelson of the US had eight bogeys and a double as he slumped to a 79.

It leaves him tied for 70th place, with only South Korea's Park Sang-hyun below him on the leaderboard.

Justin Thomas, last year's runner-up, finds himself alongside fellow American Matt Kuchar on five-under-par.

Tiger Woods, the winner of 18 WGC titles from 1999 to 2013, registered a level-par 71.

The 14-time major champion's appearance at the event is his first in Mexico, despite having been playing on the professional tour for more than 20 years.