Jimmy Walker fired a three-under 69 to move four strokes clear after the third round of the Valero Texas Open.
The world No 13, holding a one shot overnight lead over Charley Hoffman and Aaron Baddeley in his hometown, finished with back-to-back birdies to extend his advantage over Jordan Spieth and close in on a second PGA Tour title of the year.
"It's a tough golf course,” Walker said. "I thought coming into today that even par or maybe one-under would be a good score. You definitely want to find a way to extend the lead, but you have to be smart about it."
Spieth, who won his last PGA event two weeks ago at the Valspar Championship, shot a rollercoaster 71 that included six birdies. Beginning the day two strokes adrift, he gained ground with four gains in a five-hole span on the front nine and was tied for the lead with five holes to play before losing ground.
After recovering from a third bogey of the day at the 14th with a gain at the 15, Spieth over-hit his tee-shot at the 16th past the green and behind the grandstand, taking two chips to find the green before two-putting from 5-foot for a double-bogey .
The American bounced back by holing a 24-foot birdie at the 17th after hitting out of the greenside bunker with his second shot, but missed the chance to end with successive gains after seeing a 12-foot putt at the 18th roll just wide of the cup.
"I'm going to have to hit the ball better than I did today," Spieth said. "A couple of tough breaks on the back nine. All in all, I was playing a really solid round of golf, minus a couple of decisions and a couple of swings."
FedEx Cup champion Billy Horschel birdied the final three holes to get to three-under, with Chesson Hadley and Zach Johnson finishing strongly with consecutive gains to join a five-strong group a further shot back.
Tough day
Australia's Baddeley had a disastrous day, posting seven bogeys including four in a row on the front nine of a four-over 76, while first round leader Hoffman soared to a wind-blown seven-over 43 on the front nine and needed a birdie at the last to break 80.
Phil Mickelson had all sorts of short-game troubles while shooting a front-nine 40, taking two shots from the fringe in a double bogey at the fourth, bogeying the fifth after driving into a greenside bunker and missing a three-foot par putt at the seventh.
He nearly holed out for an albatross from 258 yards on the par-five 14th, but ended Saturday two-over and level for the tournament alongside Australia's Matt Jones.
“Very tough still,” Jones told Sky Sports 4. “You’ve got to be able to control your ball as best you can and I was lucky enough to make some putts to keep the round going.
“I think I’ve got to get a top three finish to get in to the Masters so I’ve got to try my best and hopefully have a low one tomorrow and I’ve given myself a great chance today which is great.”
Watch the final round of the Valero Texas Open live from 6.30pm on Sky Sports 4 - your home of golf.