Thomas' 9-under round broke Johnny Miller's record for lowest score in relation to par. Miller held the previous record after he shot 8 under at the 1973 U.S. Open at Oakmont Country Club.
Thomas posted the fifth 63 in U.S. Open history, but it is the first to take place in the third round. His historic round included nine birdies, including three straight to finish the front nine.
Thomas, who had two bogeys on Saturday, birdied Nos. 15 and 17 before he hit a three-wood on his approach shot on the par-5 18th hole to set up a short eagle putt that allowed him to get to 9 under for the round.
"It was obviously an awesome day," Thomas said. "My game has felt pretty good all week and it was just one of those (days) -- it was just trying to take advantage of the opportunities I had."