He does need Dustin Johnson to finish fifth or worse, but it'd be a moot point if he doesn't take care of his own business either way.
McIlroy himself is suffering from the same stomach complaint that saw Phil Mickelson's brother take over as caddie after Jim "Bones" Mackay was forced into a quick dash back to the clubhouse.
Playing in only his second tournament of 2017 as he battles back from a rib injury, McIlroy's approach play was as sensational as his putting as he pieced together a sparkling round of 65 to steal a march on his rivals.
Phil Mickelson, among six players to share the lead after the first round, followed up his opening 67 with a 3-under 68 and was in a three-way tie for second place with Ross Fisher (68) and Justin Thomas (66) at 135.
Fortunately for Mickelson, his brother Tim had been alerted to the problem and was on hand to take over on the bag as his sibling looked to remain in contention for a third World Golf Championship title.
"You can hit driver on every hole and with the altitude you can really try to overpower it", he said.
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"It was fun. It was the first time we were able to do that and it was fun". But he's hurting, he just hasn't been feeling well. "He's so fun to be around and he did a great job today".
McIlroy holed out from 156 yards for eagle on the 14th hole, part of a three-hole stretch Friday when he seized control at Chapultepec Golf Club. 6 and 7 to move to 7 under par.
McIlroy said: "I'm in a great position, but I felt like I could have been a few more ahead".
Thomas is seeking his fourth victory of the 2016-17 season while world No. 1 Johnson is making his first start since winning the Genesis Open at Riviera two weeks ago.
"I think the guy that keeps it under the hole the most and obviously hits it quite close is going to be the guy that wins this week".