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.
Rickie Fowler's seven-under-par on Thursday also tied the record lowest first-round score at a US Open. That's two Brits and two Americans, and no one you'd recognize if you saw them in the Tally Ho Pub & Grill a mile from Erin Hills.
Elsewhere, Masters champion Sergio Garcia finished with a 71 to stay on six under, six behind Harman. I felt really comfortable and I was enjoying being out there.
No fewer than 44 players ducked under par in the first round, and as the second round unfolded Friday, the 7,839-yard layout continued to give up birdies freely.
Then, inexplicably, he lost his putting touch.
The 39-year-old was only able to move his fourth shot a matter of inches in heavy rough over the back of the green on the par five, before hacking out sideways and taking three putts from just off the green.
"I'm looking forward to it very much".
He was at 7-under 137, two shots clear of anyone else who played early in warm sunshine on a course that getting dry and crispy. No harm, no foul.
Casey made a snowman - a triple-bogey 8 on the par-5 14th - and still broke par with a 71. His second shot drifted off the fairway, third shot was over the green in the fescue. If you get four or five, that's pretty good. The ball landed on the front of the green just soft enough that it rolled out 8 feet by the hole. So there he stood at two under and seemingly out of the picture.
A rattled Casey added another bogey at the 15th to extend his miserable run before hitting the turbo-charger and firing off five consecutive birdies.
"I'm very excited and very happy with the score", the Japanese star said.
"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", said 14th-ranked Casey. Lots of talent, just looking for that one break-out moment. 12 and 13 to fall two shots behind Brooks Koepka.
Koepka, playing the back nine first, birdied four of his first seven holes.
Day never had a chance, following his career-worst 79 in a U.S. Open with a 75. "I got in a hazard on one". I've got four weeks off now.
Not to be overlooked was Si Woo Kim, who captured THE PLAYERS Championship last month.
If you're searching for a player a bit down the leaderboard to make a push, don't overlook Martin Kaymer.
The Southport-born Fleetwood had five birdies in his round and just two bogeys.
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.
"It's just golf and it's frustrating, but I don't let it bother me". "I got off to a good start, but it sort of caught up with me as the round went on". It was only his seventh PGA Tour event this year because of a rib injury that has kept him out of action since the Players Championship and caused him to miss the BMW Championship and the Memorial.