Casey recovered from a triple-bogey on his fifth hole of the day to add a 71 to his opening 66 and set a clubhouse target of seven under par, which was later matched by Fleetwood and American duo Brooks Koepka and Brian Harman.
Some of the world's top-ranked players and pre-tournament favorites look poised to miss the cut. For starters, Fowler finally made a bogey, just missing the fairway off the tee on the par-4 11th, putting his approach over the green in a tricky lie and failing to get up and down.
However, after dropping another shot on the 15th, Casey regained his composure superbly to birdie the 17th and 18th, the latter being the second longest hole in major history at 676 yards. "They're the widest fairways we've ever played in a US Open".
Jordan Spieth sits just inside the cut-line at even-par through 16 holes, while playing partner and defending US Open champion Dustin Johnson is flirting with a missed cut at three-over.
McIlroy won the US Open with a record 16-under-par total in 2011, but was a combined 50 over par for his seven other appearances before this week and missed the cut at Oakmont a year ago following rounds of 77 and 71.
"I know it's cliche, but the last thing you need to be doing is thinking ahead, especially on this golf course".
"It was a good display, all my own fault, of what can happen if you get out of position on this golf course, which is what I did on 14", Casey said.
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The Prime Minister promised further investment and a new Mental Health Act to end workplace discrimination. She added she would continue to work for a Labour party that "once again can deserve your confidence".
"Even just trying to take my medicine is very, very hard". I was hacking around in the rough and did not feel it once. "I've got to keep the confidence going, keep the momentum and see if we can have a good weekend and have a chance on Sunday".
"I had been swinging it well and it felt really, really good a couple holes later to be picking the ball out of the hole for a birdie". "We're in a good spot". My timing was just a little bit off.
Making matters worse, however, Fowler compounded the mistake with three-putt bogeys on the next two holes, suddenly falling from out of the lead for the first time in two days.
"I've always liked when a course plays tougher".
Masters champion Sergio Garcia returned a tidy one-under 71 to leave the Spaniard four back and in contention for a second straight major.
While McIlroy and former world number one Jason Day missed the cut, amateur Cameron Champ - who turned 22 on Thursday - and Xander Schauffele were unlikely contenders on five under par, alongside Players Championship victor Si Woo Kim.