|WGC-Mexico Championship second-round leaderboard|
|-11 B DeChambeau (US); -10 E van Rooyen (SA), P Reed (US); -9 H Matsuyama (Jpn), J Thomas (US); -8 R McIlroy (NI)|
|Selected others: -5 T Hatton (Eng), P Casey (Eng); -3 L Westwood (Eng), T Fleetwood (Eng); -1 D Willett (Eng), J Rahm (Spa), S Lowry (Ire); +5 D Johnson (US); +8 G McDowell (NI)|
American Bryson DeChambeau carded an eight-under-par 63 to take a one-stroke lead at the halfway stage of the WGC-Mexico Championship.
The 26-year-old posted seven birdies in eight holes and leads on 11 under.
South African Erik van Rooyen equalled the course record on his 30th birthday with a nine-under 62 and shares second with Patrick Reed, who returned a 63.
Rory McIlroy, two ahead overnight, made a slow start but two late birdies left him three shots back in sixth.
World number four Justin Thomas, who has twice posted a 62 at the Club de Golf Chapultepec, started from the 10th and picked up six shots in five holes after holing a 34-foot eagle putt at the first, before finishing on nine under.
Van Rooyen, yet to win on the PGA Tour, had nine birdies in a bogey-free round and matches Retief Goosen's 2002 round for the lowest score by a South African in the WGC events.
With birdies rolling in all around him, McIlroy made a slow start to round two, hampered by some wayward drives and then a series of missed putts.
After eight pars he bogeyed the ninth following a plugged lie in a greenside bunker and though he birdied the 12th he gave the shot back by three-putting the next. A towering iron to four feet at the 14th gave him another birdie and some welcome long putts helped him to a round of 69.
Former Masters champion Reed was among those to eclipse him, however, a 20-foot putt at the 17th creating a fifth birdie in seven holes.
There is no halfway cut, but defending champion Dustin Johnson remains well down the field at five over, Graeme McDowell birdied the last but is eight over, while England's Matt Wallace is plus nine after three double bogeys in a 77.