Jim Furyk was on Saturday coming to terms with his achievement after carding an historic 59 on day two of the BMW Championship in Illinois.
The eight-time Ryder Cup player 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.
The 12-under-par card at Conway Farms lifted Furyk level with Brandt Snedeker for the tournament lead on 11 under and represented only the sixth sub-60 round on the PGA Tour. Zach Johnson is third on eight under.
Northern Ireland's Graeme McDowell is one over after a 73, with Rory McIlroy was dead last on 13 over par after following up his opening 78 with a 77.
Furyk told Sky Sports 3: "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 10th, 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 followed his opening 63 with a three-under 68, with five birdies offset by dropped shots at the ninth and 16th.
He 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 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.