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.
A 94-year-old man who was attending the US Open in Wisconsin has died on Friday, the United States Golf Association (USGA) said in statement. "It's the kind of golf course that if you're on point and playing well and you're in the fairways, you really feel like you can be aggressive and attack and make some birdies".
Casey started eagle-birdie and finished with two birdies over the final four holes for his 66.
Si Woo Kim, who captured The Players Championship a month ago, had a 70 and was at 5-under 139 along with a pair of U.S. Open newcomers. If there's soft conditions, maybe we tuck a hole location a little bit more, versus if we get real firm conditions and it gets windy, we're sitting there talking among ourselves saying, 'How do we tone this thing down?'
On a day in which 105 players hit 10 or more fairways, McIlroy only hit five.
"I'm feeling good", the Victorian said later.
ERIN, Wis. (AP) - The biggest surprise at this U.S. Open was not who was leading, but who was leaving.
"This is not your typical U.S. Open set-up", McIlroy said.
Jason Day, like McIlroy, failed to survive for the closing 36 holes here. "The scoring was so good this morning". I was just trying to show what we have in some spots. The next one went long, he chipped up to 12 feet and took two putts from there.
Not for the first time, Scott's caddie, Steve Williams, played his part in the last-minute revival. Will it be enough to delay play? According to a report by ESPN.com, AirSign pilot Trevor Thompson suffered serious injuries in the crash, including burns on his back, head and neck. The Canadian was 100 feet away for birdie on No. 6 and burned the edge of the cup on that one, except that it ran by some 7 feet and he three-putted for bogey.
"I was teeing off and I looked up and saw it on fire, and I felt sick to my stomach", Jamie Lovemark said.
Brooks Koepka, Tommy Fleetwood and Brian Harman were at 67, with Patrick Reed and Adam Hadwin in the group at 68.
But it got worse on the back nine, when bogeys on the 11th and 13th were followed by two double bogeys on 15 and 17. Length wasn't the issue.
He opened with a tricky par save on the par-4 10th hole, getting up and down from a sliver of tangled rough after he hit his approach past the green.
Pro tip for spectators at really any golf tournament but especially Erin Hills: If you are on crutches or a walking boot, maybe you sit this one out.
The most damaging hole was early in his round when Johnson hit iron off the tee on the par-5 14th, only to lay up in grass so thick that a marshal marked it with a tiny flag, and then it was hard to find the flag.