Former Open champion Louis Oosthuizen will take a three-shot lead over Rory McIlroy into Monday's final round of the Deutsche Bank Championship after a brilliant 63 at TPC Boston.
Oosthuizen began the day one shot behind playing partner McIlroy and watched as the world number one birdied the first two holes.
However, the South African also picked up a shot on the par-five second and then fired seven birdies in succession from the fourth.
That meant a tournament-record 29 on the front nine and although his birdie on the 10th was followed by six pars and a bogey, a closing birdie on the par-five 18th saw the 29-year-old finish 19 under par.
"Louis put on a display right there," McIlroy said. "I was delighted when I got the honour back on the 12th tee."
McIlroy returned a 67 to finish three behind on 16 under, adding: "I've come from farther behind before. It's going to be an interesting day tomorrow."
Dustin Johnson continued his quest for a Ryder Cup wild card with a 65 to move into third place on 13 under alongside Tiger Woods, who carded a 68.
Oosthuizen, who once shot a round of 57 on his home course in Mossel Bay in South Africa, said: "This felt similar, except that I was playing with friends and not in a big tournament like this one.
"Every putt had perfect speed. I told Rory, 'Sorry, but you've got to take it when you can."'
Despite being six shots off the lead, Woods has not given up hope of victory after a host of low scores this week.
"I'm going to have to put together one of those rounds," Woods said. "It won't surprise me if somebody shoots eight or nine under par tomorrow because of where the pin locations are.
"Somebody is going to go out there and do it. It may be early, it may be late, who knows? But hopefully, I'm one of those guys."