Day's meltdown began when not one, but two chips from the first cut of rough behind the fourth green hit the top of the hill, then trundled back down to nearly where Day was standing. Joined by Americans Brian Harman and Brooks Koepka on seven-under, the leaderboard for the second major championship of the year has concertinaed up meaning any of the 68 players to have made the cut for the weekend have a chance of scooping the title. Rickie Fowler, the first-round leader, is in a three-way tie at 6 under, followed by an eclectic mix of five golfers at 5 under.
The top three players in the world have never missed the cut in the same major dating to when the world ranking began in 1986.
Johnny Miller shot 63 at the 1973 US Open but it was at the par-71 Oakmont, with this week's venue Erin Hills being a par-72. Jason Day and Rory McIlroy also missed the cut, and this marked the first time the top three players in the world have all missed the cut in a major.
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.
South Africa's Louis Oosthuizen had looked like being the early pacesetter, the former Open champion going to the turn in 32 and picking up shots on the 11th and 15th, only to run up a double bogey on the 17th.
Rory McIlroy and Jason Day never recovered from theirs. Three more birdies on his inward nine took him to seven under and a piece of US Open history.
On a day in which 105 players hit 10 or more fairways, McIlroy only hit five.
"It feels great. I came in this week with no expectations really at all", said the 22-year-old. Hit a lot of shots probably better today than I did yesterday.
American Chez Reavie also scored 65 on a day when Erin Hills failed for the most part to bare its fangs.
"But this game's all about putting. I just didn't get it in the hole fast enough". The next one went long, he chipped up to 12 feet and took two putts from there.
Fowler dropped to a share of fifth at six under, alongside compatriots Jamie Lovemark and JB Holmes (both 69). "You've got to make putts. But you don't really know until you put a card in your hand and you're under the gun a little bit", said McIlroy, who said goodbye at 5 over. After another week, it's the WGC-Bridgestone and PGA Championship.
McIlroy's pre-tournament comments proved disappointingly accurate after he had predicted that anyone who could not hit the wide fairways " might as well pack your bags and go home".
"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", McIlroy said. "I'm enjoying it right now".
"I felt like I've been playing pretty well all week and didn't have quite the numbers to show for it".