A day after both Justin Thomas and Jason Kokrak equalled the Medinah course record, Hideki Matsuyama broke it by two shots with an astonishing nine-under-par 63.
In doing so, the 27-year-old Japan player also took the BMW Championship lead after round two.
Matsuyama hit nine birdies and no bogeys to move to 12-under overall, one shot in front of Americans Patrick Cantlay and Tony Finau.
"It seemed like every birdie chance I had I made the putt," Matsuyama told reporters.
Thomas shot a three-under 69 to move to 10-under overall, but fellow overnight leader Kokrak had a less successful day and fell back to six under after a one-over 73.
After going around in 66, England's Tommy Fleetwood moved up to eight under alongside Northern Ireland's Rory McIlroy, who shot 67.
Tiger Woods's form remains inconsistent, getting five birdies but also four bogeys in a second consecutive round of 71.
Brooks Koepka also scored a 71, moving the world number one to five under overall.
England's Justin Rose shot a 73 and fell back to three under, one behind his compatriots Paul Casey and Ian Poulter.
Rory McIlroy birdied the last to stay in the hunt after round two of the BMW Championship in Illinois.
The county Down man finished on eight under par after a 67, which left him four strokes behind Matsuyama.
British Open champion Shane Lowry was left in a tie for last place in a tournament where there's no cut. His round of 74 included six bogeys, as he finished on two over par.
While Graeme McDowell couldn't build on yesterday's total, as he shot a level part 72 to remain on three under par.