Needing a win and help from Dustin Johnson to regain the world No 1 ranking, the Irishman’s recent intensive bout of injury- enforced putting practice generally paid off at a tight and tree-lined Chapultepec Golf Club.
While he was struggling with a minor stomach ailment, the Holywood star showed no visible ill effects from the rib injury that kept him out of the game for 44 days as he made.
“The rib is great,” McIlroy said.
“I have been good for the past few weeks, ramping up the practice and playing my first competitive round out there today, I didn’t feel it at all. It’s all positive in that regard.
“I probably could have played Honda last week but I wasn’t going to know my body would react after four days in a row.
“I thought this a great place to start with four guaranteed rounds. I am taking next week off just to see how everything reacts and responds.
“It was the logical place to come back. It would have been great to sleep in my own bed last week but I stayed patient and the week longer will definitely pay off.”
Not only did he look pain-free, he hit nine tee shots over 300 yards on an old-fashioned but fascinating 7,330-yard track that requires as much strategy as straight hitting.
Even allowing for a 15% boost in distance due to Mexico City’s 2,250m (7,700 ft) elevation, McIlroy was impressive from the tee, solid on the greens and tidy with his short game.
He wasn’t always straight but if there were any lingering worries about how well the stress fracture in his ribs has healed, he dismissed them with a majestic eagle three at his 15th hole.
The 27-year old bombed a 368-yard drive uphill — the longest drive of the day on the hole to that point by 20 yards — on the 615-yard, sixth.
Left with a blind, 264-yard second shot over a lake to a two-tiered green, he didn’t think twice and rifled a sky-high long-iron that finished 28 feet from the stick.
McIlroy gave a mini fist pump as the eagle putt disappeared and he moved to within a shot of early pace-setter Jimmy Walker on three-under-par.
Given how much practice he has been doing on his putting during his injury layoff, the four-time major winner was pleasingly clinical on the greens, using the blade just 26 times as he hit 14 greens in regulation.
His lone bogey came at the fifth (his 14th), where he three-putted from close to 60 feet.
But he wasn’t the only one to have trouble.
Playing partners Johnson (70) and Hideki Matsuyama (72) both had their struggles with the blade, taking 30 and 33 putts respectively.
World No 1 Johnson started birdie-birdie but lost his confidence with the putter and missed four putts between three and five feet on the poa annua greens, he still made six birdies to finish the day in red figures.
Starting on the back nine, McIlroy birdied the par-five 11th from 14 feet, then saved par from six feet at the 13th, 14th and 17th before missing another putt of the same distance for birdie at the 388-yard 18th, where he had just 49 yards for his approach.
He was far more impressive on his homeward nine, missing just one green in regulation.
At the 325-yard first, he drove to the edge to the green and made an eight-footer for birdie, then missed a couple of chances before that outrageous eagle at the sixth.
Meanwhile, Open champion Henrik Stenson from Sweden withdrew with a suspected stomach virus after playing 11 holes in three-over-par.
England’s Ross Fisher and US PGA champion Walker set the early clubhouse target with four-under-par 67s. They were joined by Spaniard Jon Rahm. Englishman Lee Westwood was leading on five under par through 17 holes.