Jim Furyk became only the sixth person in US PGA history to card a 59 as he joined Brandt Snedeker in a share of the lead at the BMW Championship in Illinois.
The American’s 12 under par round at Lake Forest took him level with Ryder Cup team-mate Snedeker, who carded a 68, on 11 under at the halfway stage.
South African Charl Schwartzel leads The European Tour challenge on six under after a one under 70, with Sergio Garcia two shots further back.
Eight-time Ryder Cup player Furyk holed out nervelessly from inside three feet at his last hole, the ninth, to crown a round featuring 11 birdies, an eagle and a bogey.
Furyk said: "It was a little hectic down the stretch.
"I had the three-putt at five and you could see the nerves at the sixth, leaving the putt short.
"I got myself back on track at seven, then I was between clubs at eight with a pretty good chance to make my birdie.
"I told myself at nine, 'how many chances are you going to have in your life to do this again?'.
"It'll sink in later. I'm excited."
Starting from the tenth, the 2003 US Open Champion played the back nine in 28 - highlighted when he holed a 115 yard wedge for eagle at the par four 15th - and kept up his charge after the turn.
He made three birdies in succession from the second, although a dropped shot at the fifth threatened to derail him.
The 43 year old got back on track with birdie at the seventh, though, meaning he needed to play the final two holes in one under par.
He missed his chance at the par five eighth - although he was fortunate to avoid a deep bunker just in front of the green - but he hit his approach to the next right at the pin and drilled the short putt into the centre of the hole.
Snedeker was watching as Furyk holed out for his 59, and responded with a warm smile despite seeing his rival move level with his 36-hole total.
Snedeker and Furyk lead the tournament by three strokes at the halfway stage with Zach Johnson (70) in third on eight under.
Furyk added: "I'll have to calm down later tonight and realise I've got myself in contention at a golf tournament where I was about nine back.
"I think I'll sleep pretty good. Sundays I don't ever sleep when I've played poorly, I'm running back on the things could improve. I don't think I could improve much today so I'll sleep all right."
Tiger Woods (72) was on four under, despite being handed a two shot penalty on the first hole, making his double-bogey six into an eight. The World Number One's ball was adjudged to have moved as he attempted to remove some loose impediments before taking his third shot, having hit his second over the green and onto some dirt under a stand of trees.
Jim Furyk on Friday became the sixth player in PGA Tour history to card a 59. Here are the other golfers who have achieved the feat.