Jim Furyk shot 59 to equal the record for the lowest 18-hole score in PGA Tour history in round two of the BMW Championship in Illinois.
The 43-year-old American becomes only the sixth player to card a sub-60 round after 11 birdies, an eagle and a bogey.
Furyk's score of 12 under at the par-71 Conway Farms course gave him a share of the lead at 11 under with last year's FedEx Cup winner Brandt Snedeker (68).
"It's absolutely the best round of golf I've ever played," Furyk said.
"It will sink in later and I'll have to calm down and realise I'm in contention for the golf tournament."
Furyk's previous best round is a 62, which he has achieved five times in his career.
The eight-time Ryder Cup star joins Al Geiberger, Chip Beck, David Duval, Paul Goydos and Stuart Appleby as the only players to shoot 59 on the PGA Tour.
No player on the European Tour has ever achieved the feat, although 16 players have shot 60.
Furyk set up his round with a stunning outward nine of 28, eight under par, which included an eagle at the 15th where he holed out from 115 yards.
Further birdies at the second, third, and fourth raised hopes of a possible first 58 but he made his only mistake of the day with a three-putt bogey at the fifth.
However, he immediately recovered by sinking an 11-foot birdie putt at the seventh.
Furyk, who was a surprise omission from the US team for next month's Presidents Cup, escaped a tricky lie under the lip of a greenside bunker for a par five at the eighth.
Needing to birdie his final hole, the tricky par-four ninth, to break 60, Furyk split the fairway off the tee and struck a superb wedge from 103 yards to three feet before he calmly rolled in the putt.
"It definitely played some tricks with my head on the way in," he added.
Former Masters champion Zach Johnson was three two shots off the lead at eight under after a second-round 70, while Tiger Woods lost ground after being penalised two strokes at the first.
The 14-time major champion's ball was seen on TV to have moved as he removed some loose impediments behind his ball and video footage was reviewed as he completed his round.
Slugger White, the PGA Tour's vice president of rules and competition, advised Woods not to sign his card until he had seen the video himself and the American's six was turned into an eight for a 72 and four under.
"I told him not to turn it in until I talked to him, and he looked at it, and clearly felt like the ball just oscillated," said White. "It's pretty clear that the ball did move."
England's Brian Davis led the British challenge on three under after a 67, one shot ahead of compatriot Luke Donald (70).
Another Englishman Lee Westwood looked less troubled by the back and neck problems that blighted his first round as he improved from an 80 to a 73 to end 11 over.
Defending champion Rory McIlroy, 24, was unable to gain any significant ground, though, as he finished only one shot better off than his first-round 78.
The Northern Irishman, who is without a win since the World Tour Championship in November last year, reached halfway at 13 over and in last place after a round that included six bogeys, one double bogey and two birdies.
The BMW Championship is the third leg of four in the FedEx Cup end-of-season play-off series.
There is no halfway cut but only the top 30 players in the standings will qualify for next week's Tour Championship in Atlanta, where the FedEx Cup winner will claim the $10m (£6.3m) prize.