MEXICO CITY (AP) -- Rory McIlroy had more trouble with his stomach than his ribs. The golf was just fine.
Playing for the first time in seven weeks because of a rib injury, McIlroy coped with a bad stomach and the high altitude with a 3-under 68 that left him one shot out of a six-way tie for the lead as the Mexico Championship made a strong debut Thursday.
Phil Mickelson and Lee Westwood, two of the four players who were in the field for the first edition of this World Golf Championships event in 1999, each shot 67 and were joined at the top by PGA champion Jimmy Walker, Ross Fisher, Jon Rahm of Spain and Ryan Moore.
Westwood and Walker each made it to 6 under until two bogeys over their final few holes.
McIlroy, who can return to No. 1 with a victory this week, had not competed since Jan. 15 because of a hairline fracture of a rib. That was never a problem. Leaning on his driver as he waited to tee off midway through his round, he said he spent most of the night in the bathroom and "the altitude doesn't help."
He made the golf look easy, rarely getting out of position while playing aggressively with a driver early in the round and surging late with an eagle on the par-5 sixth. British Open champion Henrik Stenson wasn't so fortunate. He withdrew after 11 holes with a stomach virus.
The biggest surprise was Chapultepec Golf Club, hosting the best players in the world at nearly 7,800 feet. Roberto Castro hit a tee shot 407 yards. Mickelson hit one 379 yards, his longest in recent years without hitting a cart path. Dustin Johnson, in his debut at No. 1 in the world, hit 3-wood on the 316-yard first hole that bounded over the green, into the hedges and out-of-bounds. For all that length, no one could do better than 67, and only 27 players in the 77-man field broke par.