Sixty-eight players made the cut, with former champion Webb Simpson, former Masters and British Open victor Zach Johnson and former PGA champion Keegan Bradley all making it on the number.
This isn't like most Opens.
The Offaly man posted a disappointing 74, which saw last year's runner-up move from one under to one over at the end of the second round.
None of the leaders had as shaky a stretch as Casey, who made 8 on the par-5 14th hole - after hitting into the fescue and almost whiffing on his swing to get out - and bogeyed the next hole.
At the top of the leaderboard at present are Rickie Fowler and Brooks Koepka on 8-under, although both have yet to complete their rounds.
Fowler was among those who wouldn't mind seeing the conditions toughen up for the final 18 holes. Moments later, he began a run of five straight birdies that put him right where he wanted to be going into the weekend. Harman made a 30-foot birdie putt on the final hole to hold off Dustin Johnson and Pat Perez by a stroke. And he wasn't through.
Day added a 75 to his first-round 79, which included two triple-bogeys. And then Morgan came undone.
Few have had a more stomach-churning ride at a major as Casey, playing the Erin Hills back nine first, opened his round with a birdie at the 11th before plunging down the leaderboard with a triple-bogey at 14 and a bogey 15, then rocketing back into contention with five consecutive birdies.
And then, the old McIlroy kicked into gear and finished with a flurry of gained shots as he kept the ball in play off the tee and proved majestic with his mid-irons to set up several birdie chances. "Looking forward to the next two days". Tom Kite won his only major.
Rickie Fowler leads US Open on day of highs and lows
Open is known for putting a premium on par, even as the USGA tries to convince golf fans that it's not preoccupied with par. The incident happened at around 11:55am with the opening round of the season's second major under way at Erin Hills.
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The 28-year-old needed to make a fast start on Friday to avoid another early exit, but missed from five feet for birdie on the 10th - his opening hole - and 12 feet on the 11th, before a poor chip from the back of the 12th green led to a bogey.
On every hole, on every single shot, Stricker has had the crowd following him and rooting for him. No one had a chance.
Fleetwood found his way to the top with a rather ordinary four-birdie, two-bogey round. I'm glad I qualified and was able to play.
Scheffler is three shots back of the lead on a jam-packed leaderboard.
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. Even as the course slowly recovered from rain early in the week, reasonable scores were available. Players noticed firmer greens that were allowing putts to race past the hole.
The tournament was such a slog that it didn't finish 72 holes until Monday.
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.
McIlroy worked with his coach Michael Bannon on making a smoother transition from the top of his backswing and although he was three over par after 12 holes, birdies on the fourth, fifth, seventh and ninth ended the week on a high. He didn't win, either, closing with a 76 to finish at 2-under 278, two behind Lucas Glover.
English pair Paul Casey and Tommy Fleetwood are among four players with a share a lead of the US Open at the half way stage at Erin Hills.