The Northern Ireland star had two birdies and an eagle with one three-putt bogey at Chapultepec Golf Club near Mexico City.
The Northern Ireland star, competing for the first time in seven weeks after recovering from a stress fracture, had two birdies and an eagle with one three-putt bogey at Chapultepec Golf Club near Mexico City.
"The rib is fine," said McIlroy, who was just one shot behind the big group sharing the lead on four-under 67 that included five-time major-winner Phil Mickelson, rising Spanish star Jon Rahm and Englishmen Ross Fisher and Lee Westwood.
"I didn't feel it at all so it's all positive," said McIlroy, who teed off on 10 and posted two birdies before a bogey at the fifth.
He rebounded with a 28-foot putt for eagle at the par-five sixth that kept him firmly in the hunt for a third World Golf Championships victory that could see him return to number one on the world -- should current number one Dustin Johnson finish in a tie for fourth or worse.
McIlroy's only competitive appearance this season was in the South African Open, where he lost in a playoff to Graeme Storm in January.
He withdrew from the Abu Dhabi HSBC Championship with the rib injury, and also opted out of last week's US PGA Tour Honda Classic in Florida.
He said the luxury of knowing he would get four rounds guaranteed in the WGC event with no halfway cut was a lure, as was a chance to have next week off "to see how everything reacts" to the stress of playing.