Louis Oosthuizen carded his second straight six-under 66 to take a two-shot lead after the third round of the Houston Open on Saturday.
The South African, who shared the overnight lead with Brian Davis, moved into 17-under at the Redstone Golf Club.
American Hunter Mahan shot a day's-best seven-under 65 to be second at 15-under, while Swede Carl Pettersson and Englishman Davis are a further shot back.
Oosthuizen, the 2010 British Open champion, bogeyed his first two holes, but recovered to birdie eight of his last 13 on a sunny but windy afternoon in Texas.
"It's a great leaderboard behind me," Oosthuizen said.
"It's going to be tough, but I feel like I'm ready for it."
His countryman, Ernie Els, needs a win to qualify for next week's Masters, but appears set to miss the major for the first time since 1993, sitting at eight-under.
"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."
Mahan only holed one bogey in his stellar round, shooting four birdies on the front nine before repeating the effort around the turn.
American James Driscoll is fifth at 12-under, a shot ahead of countrymen Ryan Palmer, Keegan Bradley and defending champion Phil Mickelson.
The second round was completed earlier in the day after a weather delay on Thursday pushed the tournament back.
Major winners Charl Schwartzel, who will defend his Masters crown from last year, and Darren Clarke both missed the cut.
Northern Irishman Clarke is fighting to be fit for the Masters after limping off with a groin strain.