Branden Grace shot a 69 at Carnoustie to take a four-stroke lead into the final round of the Dunhill Links Championship.
The South African is 20-under overall after a remarkable 60 in the first round at Kingsbarns.
"I didn't play particularly well but managed to get it around," Grace said. "If we have a great day tomorrow, it's going to be a low scoring round, and if the weather turns bad on us and the wind starts blowing, you'll never know what St. Andrews has got for us."
Thorbjorn Olesen of Denmark is second on 16-under following a third-round 68 at Kingsbarns.
"(Grace) is a great player but I'm going to do my best," Olesen said. "I find the greens at St Andrews easy to read."
Swedes Frederik Andersson Hed (66) and Alexander Noren (65) are tied for third, one shot further back from Olesen.
United States Ryder Cup star Dustin Johnson (67) made the 5-under cut for the final round with a shot to spare on the Old Course at St. Andrews.
Germany's Martin Kaymer and Sweden's Peter Hanson both shot 69 to make the cut.
Grace led by six with just four holes left before a double bogey six on No. 15 when he drove into a fairway bunker. Olesen also had a double bogey six on the 17th at Kingsbarns.
The tournament has a first prize of $800,000 and is played over the Old Course at St Andrews, the Championship Course at Carnoustie and the Kingsbarns Golf Links.
American businessman Hugh Connerty and Thai professional Thongchai Jaidee lead the Team Championship by five shots on 33-under.
Olympic swimming great Michael Phelps, playing with pro-partner Paul Casey, failed to make the cut. On Friday Phelps holed an astonishing 153-foot putt during the second round of team play.