Thomas opened with a pair of birdies and hit back from a bogey at the fourth with an unlikely birdie at the next, holing a superb putt through the fringe which turned a full 90 degrees.
Oosthuizen, whose sole major title to date came at St Andrews in 2010, birdied the first, fourth, seventh and eighth to move to four under par, while 2015 victor Spieth had covered his first 11 holes in level par. After playing partner Jonathan Randolph made a longer putt for bogey 6, Thomas' eagle putt was center-cut for history. History, in the end, came down to eight feet.
Justin Thomas stepped up to his putt at the fifth hole Saturday at the U.S. Open and WAIT HE'S FACING THE WRONG WAY SOMEBODY TELL HIM. However, that would have to wait until the 18th as he left it low and walked away with a birdie.
Justin's closest pursuers were Tommy Fleetwood and Brian Harman at 10-under, one shot behind Thomas.
Thomas played his college golf at Alabama, and one Twitter user challenged his choice of what seemed to be a very Auburn-esque orange shirt for Sunday.
"Jimmy is it any good?"
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It bounced at the front of the green and once again rolled to about six feet, but this time Thomas would not miss the putt for eagle, 63, a place in history with the lowest ever round in US Open history.
"I wasn't as nervous as 15, I really just tried to feel it and get it in".
"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".
Justin Thomas had himself a day.
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.