Shane Lowry proved the best of the Irish and the only one of the starting quartet to make the cut, reaching the half-way stage just inside the cut mark on one over par.
While Erin Hills was still not playing as hard as many past U.S. Open courses, it was beginning to firm up from the early week rain, plus the USGA had a few tricks up its sleeve. 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.
Day never had a chance, following his career-worst 79 in a U.S. Open with a 75.
"[It's] not every day you enjoy a round of golf with an eight on the card, but I'm a pretty happy man", said Casey.
"You can not play this golf course if you're not in position off the tee and I wasn't in position", McIlroy said.
"But for me, it felt like it played tougher today".
Defending champion Johnson will aim to bounce back from a lacklustre 3-over-par 75 when he tees off alongside Jordan Spieth and Martin Kaymer in the afternoon session.
The young Englishman got another good putt to drop for birdie at the 12th, and a blocked approach to the 17th green led to another bogey before he got the shot back with an excellent four at the last. Not everyone can take advantage. He loved Erin Hills, a course with wide fairways between the knee-high fescue that he figured would allow him to attack.
"If you look at the golf course and you even talk to me, Jason or Rory, this course sets up flawless for us", Johnson said.
"I just naturally have done it since I was 15 or 16".
Fowler's seven birdies were from no more than 12 feet, including three in a row around the turn.
Willett, who pulled out before the start of his second round, was heading towards the Erin Hills exit anyway after an opening round 81.
Oosthuizen, who finished second in the US Open in 2015 and lost a play-off for the Open a month later, had to settle for a 68 to finish four under par.
Reigning Masters champion Sergio Garcia headlines a group still in the hunt at three under.
"It was still gettable", said Charley Hoffman, who has posted 70-70 and is three shots out of the lead.
For most others, like Henrik Stenson, it was being careless off the tee and facing the rigorous test of recovering.
Only Shane Lowry (71 for 145) of the four Irish starters managed to survive the cut, right on the mark in his case.
"Even though it's very disappointing to not be here on the weekend, I think these last two rounds will serve me well going into the summer, " he said. "The scoring was so good this morning". Schauffele went 26 consecutive holes without a bogey until he rammed his long birdie putt on No. 9 about 12 feet by the hole and narrowly missed the par putt. He was in California for his daughter's high-school graduation, hopeful for enough of a weather delay to jet across the country to Wisconsin.
Torrential rain that doused the 7,818-yard layout at Erin Hills overnight presented the field with a feast of birdie chances, transforming the leaderboard into a sea of red as dusk fell.