Rory McIlroy may be sick but the four-time major winner is the man to beat at the halfway stage after rising to the top of the WGC-Mexico Championship leaderboard by two strokes.

Having overcome a rib injury that sidelined him since mid-January, McIlroy has been suffering from a stomach bug at the co-sanctioned PGA and European Tour event in Mexico City.

But it has not kept McIlroy down, with the Northern Irishman nine under through 36 holes following Friday's second-round 65.

McIlroy was six under the card at Club de Golf Chapultepec, ahead of Phil Mickelson, Ross Fisher and Justin Thomas.

Former world number one McIlroy broke his round open with a thrilling eagle on the par-four 14th hole. He hit his second shot left of the pin, then watched as it spun down the slope into the centre of the cup.

Mickelson, who played with his brother on his bag after Jim Mackay left due to the flu, is joined by Thomas and Englishman Fisher at seven under.

Among six players tied for the lead on Thursday, Mickelson and Fisher lost top spot after shooting a three-under-par 68s.

Thomas rose 12 positions thanks to his 66, which included eight birdies and three bogeys.

World number one Dustin Johnson shook off the putting woes that haunted him during the first round to shoot a five-under-par 66.

Johnson is three shots adrift alongside Andy Sullivan (65) and Daniel Berger (66).

Europeans Martin Kaymer (67), Tyrrell Hatton (67) and Thomas Pieters (69) are still in contention at five under.

Rickie Fowler, who is trying to copy Adam Scott by backing up a win at the Honda Classic with a victory at this event, is four under following successive 69s.

Meanwhile, two-time major champion Jordan Spieth recorded a 72 to be one over the card heading into Saturday's penultimate round.