Tiger Woods, in his first tournament appearance in Mexico, received a boxing announcer-like introduction at the first tee, bringing a broad smile to the 14-times major champion's face.
That grin promptly disappeared, however, when Woods hit his drive out-of-bounds. His reload was almost as bad, though it stayed in play and he did well in the end to salvage a double-bogey on the way to a 71.
Phil Mickelson's title defence was left in tatters after he opened with a 79, leaving him 16 shots behind McIlroy.
The leader has not won in almost 12 months but has had top-five finishes in all three of his starts this year.
"Winning is a byproduct of doing all the little things well and I feel like I'm on a really good journey of doing that and I think it's a matter of time," he told reporters.
McIlroy, who hit 17 greens in regulation, said he could hit his driver 370 yards in the rarefied air this week, more than 100 yards further than in the cold at sea level at Riviera last week.
"It's fun," he said. "I'm glad you don't have to do it every week but while we're here might as well make the most of it.
"I had three really good putting rounds at Riviera. I got a
little bit of confidence and it was nice to see that carry on to this week as well."
His near ace came at the 305-yard par-four first, his 10th hole, where he hit a "cut two-iron" that landed softly on the front of the green.
His ball almost grazed the hole as it trickled past, before settling six feet beyond and McIlroy converted the putt for an eagle two.
Woods was four shots worse off at the hole.
"I pulled across it to try to cut it and hit it dead off the toe," Woods said of his opening tee shot.
"(The next shot) was on the same exact line. (I was thinking) this could be a pretty good number."
(Reporting by Andrew Both in Cary, North Carolina; Editing by Ken Ferris/Peter Rutherford)
