McIlroy’s only previous appearance in 2017 had come in the South African Open in January, when he lost out to Graeme Storm in a play-off despite suffering from what was subsequently diagnosed as a stress fracture to his ribs.
The 27-year-old still arrived in Mexico City knowing that he could reclaim top spot in the world rankings for the first time since August 2015 by winning his third WGC title, as long as current number one Dustin Johnson finishes joint fourth or worse.
And the four-time major winner was ideally placed to fulfil his part of the bargain after a second round of 65 at Chapultepec Golf Club which left him two shots ahead of Phil Mickelson, Justin Thomas and England’s Ross Fisher.
Johnson carded a 66 to end the day in a share of fifth place with Andy Sullivan and Daniel Berger, Sullivan holing his approach to the ninth for an eagle before adding birdies on the 11th, 15th and 16th in a superb 65.
“Overall great,” was McIlroy’s reaction in a post-round interview broadcast on Sky Sports. “Going out starting the day I would have taken a 65, but I had a great chance for birdie on 16, three-putt 17 and then miss from short range on 18.
“I’m in a great position, leading going into the weekend, but I feel like I could have been a few more ahead.
“I tried to approach this week not with low expectations, but just to see how it went, take each day as it came and through the first two days I’m in a good position.
“My body feels good, which is the most important thing I think, and if I can continue to play the way I have I’ve got a great chance this week.”
McIlroy started the day by driving the green on the short par-four first before three-putting for par, but then birdied the second, fourth, sixth and ninth to reach the turn in 31.
After a bogey on the 12th, McIlroy hit back in style with a birdie on the next before enjoying a stroke of luck on the 14th, where his approach landed 15 feet left of the flag but kicked back onto the green and rolled into the cup.
There was nothing fortunate about another birdie on the par-five 15th, but the Northern Irishman then three-putted the 17th - and threw his ball into a greenside water hazard in frustration - before seeing his close-range birdie putt on the last lip out.
Mickelson had earlier showed few ill effects after being forced to change his caddie early on day two, with Jim Mackay lasting just three holes before being forced to leave the course due to a stomach virus.
Fortunately for Mickelson, his brother Tim had been alerted to the problem and was on hand to take over on the bag, with the pair combining for a 68 containing four birdies and a solitary bogey.
“He’s irreplaceable, he’s one of the best in the business,” Mickelson said of Mackay. “He’s so good at club selection and here at altitude that’s where he is invaluable, but he’s hurting.
“But on the positive side I had a lot of fun with my brother. He’s a good player in his own right, a scratch player who understands the game and has caddied for many top players.
“He does give me the needle but I think that’s so funny and keeps me relaxed. He has a few stories about me that I wish he didn’t which can be challenging, but he’s fun to be around.
“As a college golf coach for many years he understands smart strategy and he was there if I needed to talk through some stuff.”
Russell Knox is tied for 60th after rounds of 73 and 74, the latter consisting of three birdies and six bogeys.
Elsewhere, American Michelle Wie will take a two-shot lead into the final round of the 10th HSBC Women’s Champions event in Singapore.
Wie, who’d led after the opening round, regained the upper hand at Sentosa Golf Club with a third-day 67 to sit on 14-under-par.
World No 1 Lydia Ko (67), second-ranked Ariya Jutanugarn (69) and Korea’s Sung Hyun Park (68) are her closest challengers.
Scottish No 1 Catriona Matthew leapt 14 spots to a share of 17th position after a 67 to sit on seven-under.