Rory McIlroy produced a strong finish to his second round at the WGC-Mexico Championship but still lost ground at Chapultepec Golf Club.
The world number one held a two-shot overnight lead but, with low scoring the order of the day, the Northern Irishman found himself one over par 13 holes into his round.
Three birdies in the last five holes saw McIlroy sign for a two-under-par 69, with his eight under total three shots behind American leader Bryson DeChambeau.
"It was a lot of fun," DeChambeau said after coming home in 29.
"Making those putts the way I did, striking the ball the way I did, it's surely a joy out there.
"The confidence has got to be high. It is high and it's a lot of fun to see putts finally going in. Overall everything is going pretty well, firing on all cylinders."
Having kicked off his round with eight consecutive pars, McIlroy bogeyed the ninth but holed from 21 feet at the 12th to get the shot back.
A three-putt from 10 feet at the 13th cost him another dropped shot but the four-time major winner produced a superb approach at the 14th before holing from 16 feet at the 16th and 24 feet at the 17th to keep himself in contention.
"There's a long way to go," McIlroy said on quotes reported on the European Tour website.
"I just stayed patient and try to bide my time and made a couple of nice putts coming in, and I guess my patience was rewarded a little bit."
DeChambeau made six gains in seven holes from the first – his 10th – en route to an eight-under 63 and 11 under halfway total.
That was one ahead of compatriot Patrick Reed (63) and South Africa's Erik Van Rooyen, who equalled the course record with a flawless 62 on his 30th birthday.
"The first thing I told my wife this morning: 'Man, I feel really old'," Van Rooyen said in quotes reported on the European Tour website.
"But I guess I'm still quite young and what a way to kick off the 30th and shoot nine under. A little bit of a gift to myself.
"I've been working really hard on the game, as we all do, and things came together nicely today, so I'm happy."