Louis Oosthuizen recovered from a dreadful start in his third round to go into the final day of the Houston Open with a two-shot lead.
The South African bogeyed the first two holes but bounced back to shoot a six-under-par round of 66.
He carded eight birdies between the sixth and the 17th to go 17 under, two shots clear of American Hunter Mahan.
Mahan went round in 65, while England's Brian Davis and Swede Carl Pettersson remain in contention at 14 under.
The impressive Davis did not drop a stroke for the second round in a row, carding three birdies for a three-under 69. Pettersson, meanwhile, was five under for the day.
James Driscoll is fifth on 12 under, while defending champion Phil Mickelson is among a group of three players on 11 under.
Oosthuizen told www.pgatour.com: "It's a great leaderboard behind me. It's going to be tough, but I feel like I'm ready for it."
Play began early on day three with 70 players yet to finish their second rounds on the back of bad weather on Thursday.
Jeff Maggert was the 36-hole leader on 12 under, but it could have been better had he not dropped a shot on the ninth - his last, having started from the 10th - for his first bogey of the tournament.
That was a sign of things to come for the American as he carded six bogeys, leaving him eight under overall in a tie for 14th, alongside, among others, Ernie Els and England's Lee Westwood.
Els, who said: "I needed to get to 10 or 11 under to really have a shot.
"I need a 62 or 63. It's tough to do on a Sunday, but you might as well give it a go."