Thomas matched the feat by splitting 11 (of 14) fairways, hitting 14 greens in regulation and taking 25 putts.
Thomas shot a front nine score of 31, with six birdies and one bogey.
Scoring has been surprisingly low all week, but Thomas' nine birdies and an eagle helped him become just the second player in U.S. Open history (Johnny Miller 1973) to shoot a 63. However, that would have to wait until the 18th as he left it low and walked away with a birdie.
Thomas led by two when he signed his card, although Tommy Fleetwood and Brain Harman duly closed the gap to one.
"I didn't know (what eagle putt on 18 meant)".
The Thomas' have a tradition that every time Justin does something special on the golf course like make a hole in one or shoot 59, like he did earlier this year at the Sony Open, Mike Thomas puts the golf ball in a special frame. I had 310 yards to the hole, but it was downwind so I knew if I hit it solid, I could definitely get it there.
"All we were trying to do was give ourselves a best chance to make a four, get out of there and post a great number, so those things are bonuses".
It is because of Miller's rough-around-the-edges personality that golf fans on Twitter had a field day with Thomas breaking his record. "I mean, it's all pretty self-explanatory, I guess, in terms of what it means".
Jack Nicklaus and Tom Weiskopf shot 63 in the first round of the 1980 U.S. Open at Baltusrol in New Jersey, while Vijay Singh shot 63 in the second round of the 2003 Open at Olympia Fields in IL.
"Regardless, I'm going to have to play a good round tomorrow", Thomas said.