"You don't get many rounds at the U.S. Open that are stress-free", Fowler said.
Day added a 75 to his first-round 79 which included two triple bogeys.
Fowler's seven birdies were from no more than 12 feet, including three in a row around the turn.
Casey took his place amongst the leading group after recovering from an early implosion that sent him plunging down the leaderboard.
He had a chance to extend his lead to two but missed an 11-footer for birdie on 18. Rickie Fowler, who led after Round One, is in fifth at -10.
"I don't know", Thomas said when asked what he'd say to Miller, "but I'll be pretty happy". "It was a bit of a roller coaster". The two-time major champion is 3 under for his third round at the U.S. Open and has moved into the top 25 at Erin Hills.
Rory McIlroy missed the cut for the second consecutive year and will not feature on Saturday.
However, a number of players gave impressive performances to keep up the chase at the top, with Paul Casey, Si Woo Kim and Xander Schauffele all positioned to strike. When it was clear he had nothing going in the second round, he was simply trying to stay out of the way. McIlroy was 4 shots off the cut line and Day a whopping 9 shots removed. Because of that you can go out and play some great golf but you have a bunch of guys out there that can play some good golf as well.
While Fleetwood acknowledged that winning on Sunday at Erin Hills would be life-changing, he didn't want to get too far ahead. The other four 63's were all shot on par 71 courses. That still didn't appear to be almost enough. He is 1 over for his round with six holes.
World No.9 Rickie Fowler (68) is solo fifth, with Players Championship victor Si Woo Kim, from South Korea, a shot back in sixth after also carding a 68. Cameron Champ, who just finished his junior year at Texas A&M, birdied his last hole for a 69.
Casey then reignited his charge birding his eighth hole and ended the run with a "3" at his 12th.
His problems started when he tried to lay up and round the right rough.
And he still has a long way to go. "Even just trying to take my medicine was very, very hard. It's a good 8 in the end".
American Patrick Reed also flirted with the US Open record, signing for a seven-under 65 to headline a pack of five at eight-under. Even as the course slowly recovered from rain early in the week, reasonable scores were available. He got that up-and-down for a bogey and a 68.
Still to be determined was who made it to the weekend, and where the cut would fall.
Similar scoring had initially appeared a possibility at Erin Hills, which became the longest course in major championship history at 7,845 yards in the first round and featured such thick fescue grass that patches had to be chopped down on four holes on Tuesday.
McIlroy and Day needed low rounds to have any chance of advancing to the weekendm and neither was able to produce anything near what was necessary. Day had no answers. And none of them was particularly close on a first-time major championship layout that has played soft.