Matt Kuchar fired a third-round 68 to open up a four-shot lead going into the final day of the Shell Houston Open.
The American had started the day one shot behind Sergio Garcia but, after carding a flawless second-round 65, the Spaniard found the going much tougher at the Golf Club of Houston on Saturday.
A one-over 73 left Garcia in a tie for second with Cameron Tringale (69) on 11-under as Kuchar carded the joint-best round of the day to pull clear on 15-under with 18 to play.
Phil Mickelson, meanwhile, continued his preparation for The Masters with a level par 72. Again showing no signs of the pulled muscle which forced him out of the Valero Texas Open last week, the three-time Augusta champion mixed four birdies and four bogeys and even though he fell nine shots behind leader Kuchar, it was still enough to keep the left-hander inside the top 10.
Kuchar instantly reversed positions with Garcia when play resumed earlier than scheduled on Saturday, the tee-times moved forward several hours due to bad weather forecast for later in the day.
Garcia's approach to the opening hole came up short of the green in a bunker and the Spanish Ryder Cup star was unable to save par from a poor lie, leaving Kuchar to hole from six feet for birdie to assume the lead.
A birdie from seven feet on the third briefly got Garcia back on level terms, but world number 11 Kuchar quickly responded with a superb pitch to the par-five fourth to card his second birdie of the day.
Kuchar, who had hit all 18 greens in regulation in his opening 66, also birdied the fifth from 10 feet to open up a two-shot lead, but then dropped only his second shot of the week on the ninth when his tee shot came up well short of the green on the 230-yard par three played into the wind.
Garcia did well to two-putt for par from 80 feet and remain 12 under, one behind Kuchar heading into the back nine.
The Spaniard has had 12 consecutive top-20 finishes worldwide since late last season, including wins in Thailand and Qatar, but his chances of another victory suffered a major blow with back-to-back bogeys on the 10th and 11th.
The 34-year-old was left shaking his head in disbelief when his approach to the 10th pitched near the flag and bounded over the green, but he only had himself to blame after driving into water on the next.
In the end he did well to salvage a bogey from a greenside bunker there and holed from seven feet for birdie on the 12th, only to see Kuchar's approach to the 12th spin to a halt just inches from the hole.
Kuchar's birdie took him to 14 under par and three ahead of Garcia and Tringale and he increased that lead to five with further birdies at 13 and 14.
Still five clear playing the difficult par four closing hole, Kuchar seemed to have done the hard part by reaching the green in two but he three-putted to give the chasing pack a little extra hope.
Garcia admitted he had suffered a difficult day but insists he is still in contention.
"Obviously I didn't play as well as yesterday, that was difficult to match, but I didn't feel I played that badly," he told the PGA Tour website. "There were a couple of bad shots, the tee shot on the 11th and the second shot on the 15th, but other than it was not too bad. Matt played really well in the conditions, it was quite breezy and as firm as the greens are it was difficult to hit it close."
Aussie Matt Jones will start Sunday in solo fourth after a 71 left him at nine-under - six behind - while former Open champion Ben Curtis and youngster Rickie Fowler are a stroke further back.
Fowler matched Kuchar's Saturday low 68 to leap up 13 places to tied fifth.
Former world number one Rory McIlroy suffered a disappointing day, the 24-year-old carding three bogeys on the back nine to drop down to one under par and in a tie for 37th.