Casey's opening 66 had left him a shot off the pace and he swiftly joined Fowler in the lead with a birdie from close range on the 11th, his second hole of the day, only to bogey the next and run up a triple-bogey on the 14th.
Both players liked the course, but put their performance down to a lack of execution.
Overnight leader Rickie Fowler looked set to retain his place at the top of the leaderboard but broke his 28-hole streak without dropping a shot with three bogeys in as many holes to derail his campaign.
Thomas, decked out in fluorescent pink trousers, was on the move quickly with a birdie-birdie start and had an even hotter finish to his front nine, carding birdies on four-of-five closing holes, including the last three to move to seven-under.
Casey, who is still seeking his first major championship win, was tied with Fowler atop the leaderboard when he reached the clubhouse, before the American - one of the afternoon starters - pulled clear again early in his second round.
Zach Johnson holed an 8-foot putt on the par-3 ninth for his third birdie of the front nine.
Of the four, Fleetwood - who was ranked 188th in the world rankings as recently as last August, but now up to 32nd - is the only one never to have won in the United States.
McIlroy however, who is returning from a hip and back injury, put a fearless face on his second consecutive missed cut at a US Open following his exit at the halfway stage in 2016. He still had his worst score in a U.S. Open. Day never got going, shooting rounds of 79 (including two triple bogeys) and 75 to fall nine shots short of making the cut.
Casey has recorded three consecutive top-six finishes in the Masters, but has just one top-10 in the US Open in 13 attempts, which came a decade ago at Oakmont.
The U.S. Open is hailed as the most hard tournament in golf.
However, plenty of others were firmly in contention at the start of the penultimate day; only eight shots separated the leaders from those who made the cut on the bubble.
The world number two, who earlier in the week suggested players unable to find the fairways should "pack their bags", is now contemplating an early exit following a second-round 71. Chinese prodigy Li Haotong meanwhile is the first golfer from mainland China to ever play in the third round of a major. "The golf course is actually really attractive". Casey also dropped a shot on the 15th but regained his composure to birdied the 17th and 18th - the latter playing 676 yards - to get back to four under par. "There's no secret that I'm very fiery and a competitor and I've let my emotions get the best of me for sure so I'm understanding that if I can control those I certainly have a better chance of playing well".
Matsuyama tied a second-round U.S. Open record in relation to par with his seven-under score, and his bogey-free 65 could have been even better. The last time Day missed a cut in a major was the 2012 PGA Championship, just a month after his son was born. "So that kind of takes the advantage away from the bombers".