Chicago - Jim Furyk became just the sixth player in US PGA Tour history to fire a 12-under 59, his sizzling round lifting him into a share of the BMW Championship halfway lead.
Furyk capped his round at the par-71 Conway Farms Golf Club with a three-foot birdie at the ninth hole, his last of the day.
He had 11 birdies, one eagle and one bogey and joined Brandt Snedeker atop the leaderboard.
"There's not much I could have improved on today," said Furyk, who got off to a sizzling start with birdies at his first three holes, the 10th, 11th and 12th. Another birdie at the par-five 14th was followed by an eagle at the par-four 15th, where he holed a wedge from 115 yards out in the fairway.
Birdies at 17 and 18 saw him make the turn in 28. From there, the eight-time Ryder Cup player said, he tried to forget about his first nine holes and just focus on his inward run.
He birdied three in a row starting at the second before his only miscue of the day, a bogey at the par-four fifth.
"It was a little hectic down the stretch," admitted Furyk, who hit all 14 fairways and 17 of 18 greens in regulation.
"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 kind of between clubs at eight with a pretty good chance to make my birdie."
Needing a birdie at nine to break 60, Furyk said he told himself "how many chances are you going to have in your life to do this again?"
Furyk fired his approach right at the pin, and rolled the short putt into the heart of the cup to finish his 12-under round.
Furyk is the first US PGA Tour player to post a 59 since 2010, when both Paul Goydos and Australian Stuart Appleby achieved the feat.
David Duval, Chip Beck and Al Geiberger have also done it.
However, Furyk said he couldn't afford to focus on that achievement, with the final two days of the tournament looming.
"I'll have to calm down later on tonight and realize that I've got myself in contention in a golf tournament where I was, whatever, nine back starting the day," he said. "I still have a good weekend ahead of me. I'm excited for that."
Furyk, who opened with a one-over 72 on Thursday, is now tied with Snedeker (68) on 11-under 131 - although he said he had to ask someone in the scoring tent to confirm that he had grabbed a share of the lead.
He and Snedeker are three strokes in front of Zach Johnson, who carded a 70 for 134.
Jordan Spieth, whose 65 matched the second-lowest round of the day, led a group on 136 that also included Nick Watney (69), Kevin Streelman (70), South African Charl Schwartzel (70), and Ryan Moore (69).
Only the top 30 in the FedEx Cup standings after this week's tournament will advance to next week's Tour Championship, where the playoff prize of $10m will be up for grabs.
Those who play their way into the top five in the standings going into Atlanta control their own destiny, and can claim the bonus with a victory as Snedeker did last year.
World number one Tiger Woods, who came into the week second in the playoff standings behind Henrik Stenson, was less well positioned going into the weekend than he expected to be.
His one-over 72 included a two-stroke penalty that made for a quadruple-bogey eight at the first hole.
Rules officials said they were alerted by a freelance videographer that Woods' ball had appeared to move when the American tried to move a loose impediment behind the first green.
Officials said Woods thought the ball had only oscillated, but the video showed otherwise. He was in a group on 138, seven shots off the pace.
Leading second-round scores on Friday in the $8m BMW Championship, third event in the US PGA Tour playoffs (USA unless noted, par-71):
131 - Brandt Snedeker 63-68, Jim Furyk 72-59
134 - Zach Johnson 64-70
136 - Nick Watney 67-69, Jordan Spieth 71-65, Kevin Streelman 66-70, Charl Schwartzel (RSA) 66-70, Ryan Moore 67-69
137 - Jason Day (AUS) 71-66, Jimmy Walker 72-65, Roberto Castro 68-69, Steve Stricker 66-71
138 - Sergio Garcia (ESP) 70-68, Tiger Woods 66-72
139 - Brian Davis (ENG) 72-67, Brendon de Jonge (ZIM) 71-68
140 - Luke Donald (ENG) 70-70, Matt Jones (AUS) 69-71, John Merrick 67-73, Adam Scott (AUS) 67-73, Gary Woodland 68-72, Rory Sabbatini (RSA) 69-71, David Hearn (CAN) 72-68
141 - Keegan Bradley 74-67, Hunter Mahan 68-73, Bae Sang-Moon (KOR) 70-71, Webb Simpson 72-69
142 - Bubba Watson 71-71, Daniel Summerhays 72-70, Charles Howell 71-71, Billy Horschel 73-69, Ernie Els (RSA) 71-71, Justin Rose (ENG) 71-71, Henrik Stenson (SWE) 72-70