South African Louis Oosthuizen posted a second straight 66 on Saturday to take a two-shot lead over Hunter Mahan after three rounds of the US PGA Tour's $6 million Houston Open.
Oosthuizen bounced back after two early bogeys, stringing together four straight birdies on the back nine as he took a 17-under total of 199 into the final round.
"It's a great leaderboard behind me," Oosthuizen said. "It's going to be tough, but I feel like I'm ready for it."
Mahan carded a 65 for 201 as players put in another long day on the Redstone course to get the tournament, delayed by thunderstorms on Thursday, back on schedule.
England's Brian Davis shot a 69 and Carl Pettersson of Sweden a 67 to share third on 202.
James Driscoll was alone on 204 after a 71, while defending champion Phil Mickelson posted a 70 to lead a group on 205 that also included Keegan Bradley (69) and Ryan Palmer (66).
Three-time major champion Ernie Els, who must win to qualify for the Masters next week, was in a group on eight-under.
The South African, who hasn't missed the Masters since 1993, didn't think he'd given himself much of a chance.
"I needed to get to 10- or 11-under to really have a shot," Els said. "I need a 62 or 63. It's tough to do on a Sunday, but you might as well give it a go."
Seventy players resumed their second rounds on Saturday morning.
Oosthuizen completed a 66 that put him on 11-under, one shot behind second-round leader Jeff Maggert.
Oosthuizen's third round got off to a rocky start with bogeys at the first and second.
The 2010 British Open champion drained an 11-foot birdie putt at the sixth, and added birdies at eight and nine to make the turn at 12-under.
"After that, I felt really comfortable," Oosthuizen said. "I made really good swings after that."
Mahan, who has three top-10 finishes in five previous starts here since 2007, moved up the leaderboard with four birdies in the space of six holes.
A whopping 90 players made the 36-hole cut at two-under par or better when the second round wrapped up Saturday morning.
Another cut was made after the third round, leaving 70 players in the field for the final round.