The Southport-born Fleetwood had five birdies in his round and just two bogeys.
But only for a couple minutes. More significant, though, is his own history.
Thomas' 63 is the first 9-under-par round in U.S. Open history, eclipsing Johnny Miller's 8-under 63 in 1973. Regardless of the weather, it didn't take long for the 24-year-old Thomas to understand the significance of his record round.
Also playing in the afternoon was Dustin Johnson, the defending champion and No. 1 player in the world, who opened with a 75. It would have been the round of the day had Thomas not shot 63. "It means I have a lot better chance to win the tournament than I did when the day started", Thomas said, according to Sporting News.
There are 23 golfers within four shots of the lead and 42 players under par. The last player to accomplish the feat was Vijay Singh in 2003 at Olympia Fields.
Thomas broke the U.S. Open record for low round in relation to par.
Holmes, whose best career finish at the U.S. Open is a tie for 17th in 2014, is one of the bigger hitters on the PGA Tour.
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It didn't work all that well. Fleetwood climbed the hill, opened up his wedge and flopped it to 6 feet; a potential blowup turned into a birdie that put him at 12 under.
Oosthuizen, who lost a play-off for the Masters to Bubba Watson in 2012, carded his fifth birdie of the day on the 11th to get within two shots of the lead, with the final group still more than an hour away from teeing off.
However, while the world's top three are making other plans for the weekend after missing the cut, college amateur Cameron Champ is proving that massive tee shots can still help get the job done on the long, links-style layout.
Casey flew the back of the green with his third shot and finding deep rough but could only move his ball some six inches. He finished with a 7 on the par 4, dropping back to 5 under.
Among the early starters on another steamy day at Erin Hills, Casey survived a hair-raising round that included a gut-wrenching triple-bogey and a thrilling run of five straight birdies and ended with him signing for one-under 71.
It sounds as if Erin Hills is becoming more challenging with each passing minute. He is a third-generation pro, with his grandfather Paul having played in the U.S. Open and his father, Mike, the head pro at Harmony Landing Country Club in Goshen, site of Justin's shrine. Rickie Fowler accomplished the feat in the first round, and Hideki Matsuyama and Chez Reavie hit the number Thursday. He missed a short birdie putt on 18.
Patrick Reed showed up in his Ryder Cup trousers, fired a 7-under 65, at that point tied for the low round of the tournament, and he trails by 4 shots.
What would winning the U.S. Open do for him?
"If someone shoots 9-under tomorrow in the top few (players), then I will have to shoot 10-under, I guess", Fleetwood said.