Thomas, 24, lit up Erin Hills during his third round as nine birdies and an eagle helped him break the record for the lowest score to par in the tournament's history.
But even with his historic round, Thomas, 11-under for the weekend, doesn't hold the lead. Brain Harman has a one-shot lead at 12 under.
World No.9 Rickie Fowler, who held the halfway lead, earned solo fifth with a 68, with Players Championship victor Si Woo Kim a shot back alone in sixth after also carding a 68. Winning the U.S. Open is something he has thought about many times, the 26-year-old from Southport said.
Thomas has all the momentum, though. The crowd at Erin Hills erupted once again when he dropped the eight-foot eagle putt that remained a few moments later. So there he stood at two under and seemingly out of the picture. He actually aimed his putt away from the hole and let it filter down to the hole, sinking one of the crazier putts you'll ever see.
"Not every day you enjoy a round of golf with an eight on the card, but I'm a pretty happy man", Casey said after his round.
"Just trying to play my golf tomorrow and that's it".
"I showed up for the last six holes at least", joked McIlroy, who was three over par after 12 holes before carding birdies on the fourth, fifth, seventh and ninth. He missed a six-foot putt on his last hole, which would have given him sole possession of the lead.
Johnson also missed the cut at the Memorial, so this was the first time since 2013 (Pebble Beach, Riviera) that he has missed the cut in consecutive events. There's no doubt he'll surpass T26, but can he stay consistent and finish strong enough for a surprise win? But at 28, Fowler is the ninth-ranked player in the world and should be more than ready for Sunday.
"I played a lot", a smiling Harman said.
Of the five other players who shot a 63 in the third round at a major, none went on to win. He's twice been sidelined this year due to a rib injury, and had played just the Players Championship since the Masters. If he keeps up that consistency, Kim could easily take home the title.
Brooks Koepka started on No. 10 and made four birdies on his front nine to get to 9 under, moving into a tie with Fowler for the top spot.
Casey said he would not change his approach as he pursues his first major.