Justin Thomas took command of the BMW Championship with a superb 11-under 61 to shatter the Medinah course record and open up a six-stroke lead heading into the final round.
The American had produced a first-round 65 in Illinois to equal the record along with Jason Kokrak, before Japanese player Hideki Matsuyama then shot an impressive nine-under-par 63 on Friday.
However, while Matsuyama faltered with a bogey-heavy 73 to drop out of contention at 11-under, Thomas signed for a 61 – which included two eagles and could have been even lower but for a dropped shot at the par-four sixth.
The world number 10, USPGA Championship winner two years ago, heads into Sunday at 21-under, with compatriots Tony Finau and Patrick Cantlay in joint second place after their respective 68s.
“It’s just one of those freaky days when you get in the zone,” Thomas told the Golf Channel.
“I hit the ball pretty unbelievable. It’s not like I made any long putts. I hit it close to the hole and took advantage of some opportunities when I had them and got a couple good breaks.
“That’s the stuff that happens when you shoot 11 under.”
Slovakian Rory Sabbatini sits at 14-under following a 67, picking up six birdies on the back nine.
Spain’s Jon Rham was a big mover after a 66 left him at 13-under, one clear of American Brandt Snedeker, who is in a group on 12-under with Rickie Fowler a stroke further back after a 68.
Tiger Woods, meanwhile, sank five birdies for his 67 after successive rounds of 71 to climb to seven-under.
Northern Ireland’s Rory McIlroy is at 10-under after a 70, alongside Englishman Tommy Fleetwood, who hit a double-bogey five on the second hole before recovering.
World number one Brooks Koepka dropped down the standings after a 72, two bogeys leaving him at five-under overall.
England’s Justin Rose shot a 73 and fell back to two-under, but compatriot Paul Casey’s 67 saw him climb to nine-under.