World No.3 Rory McIlroy has contended with a stomach bug to make a superb start to the World Golf Championship event in Mexico City.
In his first competitive round since sustaining a stress fracture to his rib in early January, the Northern Irishman carded a three-under-par 68 at the 77-man tournament on Thursday.
"The rib is great actually. I didn't feel (pain) at all, so it's all positive," said McIlroy.
The 27-year-old shares seventh place, one shot back from a six-way tie for the lead featuring five-time major winner Phil Mickelson and former world No.1 Lee Westwood.
The challenge for four-time major champion McIlroy wasn't Club de Golf Chapultepec's altitude of 2400 metres above sea level, rather an extended period of vomiting overnight.
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"I was up from about 3am, just worshipping the porcelain bowl," said McIlroy.
"I'm definitely a little weak, but hopefully it's just a day thing and it will pass and I'll wake up feeling a bit better tomorrow."
Paired with McIlroy was Dustin Johnson, and in his first tournament as world No.1 the American negotiated a misfiring putter to sign for a creditable 70.
Johnson sits in 20th place, while world No.4 Hideki Matsuyama rounded out the marquee group with a 72.
Defending champion and world No.7 Adam Scott made a sluggish start with a 73, sharing 47th courtesy of an uncharacteristically poor round that included five bogeys and a double bogey.
Marcus Fraser took the leading Australian honours after day one with a 71 enough to share 28th at even par, while Sam Brazel (tied 61st) struggled to a 75.
Scott Hend (tied 68th) sits at five over after a disappointing 76, while Victorian native Matt Griffin languishes near the bottom of the field after a 78.
