All at seven under par at Erin Hills.
Fowler dropped to a share of fifth at six under, alongside compatriots Jamie Lovemark and JB Holmes (both 69).
As for why Johnson, McIlroy and Day struggled, one possible reason is that all have battled injuries or personal situations over the first half of the season that may have kept them from being tournament-ready. Sergio Garcia might shoot 63 on Saturday and give himself a chance to win the first two majors of the year, but everyone here at the course will be waiting for a plague of locusts or frogs at that point!
Despite having a huge power advantage over most of the field, and displaying excellent form with his irons, Koepka was unable to capitalise on a few good birdie chances.
Rory McIlroy missed the cut for the second consecutive year and will not feature on Saturday.
Rory McIlroy at least had the satisfaction of four birdies in his last six holes to improve by seven shots on his opening 78, while Jason Day's 75 left him 10 over and world number one and defending champion Dustin Johnson finished four over. We expected some of golf's biggest names, and longest hitters, to tear it apart.
It was a nice comeback for Casey after he triple-bogeyed the par-5 14th and shot 38 on his first nine. "And unfortunately with where I was hitting it, I wasn't in the way". But then he three-putted from long range on the 13th and the 14th, and lost all hope when he missed the green on the 17th while going after the flag. "Hopefully I've got a lot of the bad stuff out of my system yesterday and some parts today. So, all in all, not bad".
The Englishman eventually signed for one-under 71 and walked into the clubhouse level with overnight leader Rickie Fowler, who had yet to start his round.
Fowler, the world number nine from California, revelled in benign conditions at Erin Hills Golf Course to reel off seven birdies and 11 pars in a superb 65.
Japan's Satoshi Kodaira was making the biggest move of the day, opening with back-to-back birdies and adding two more at 11 and 12 to reach three-under for the championship, four back of the pacesetters. No harm, no foul.
The par-4 15th was set up as a drivable hole - downwind and only 288 yards. He failed to advance his fourth shot more than a foot as the high grass grabbed the club. So there he stood at two under and seemingly out of the picture.
It was day of sizzling drama on the links-style layout that featured stunning birdie runs and spectacular collapses - some like Casey coming on the same scorecard.
"Not every day you enjoy a round of golf with an eight on the card, but I m a pretty happy man", Casey said.
"I started to let it go a little bit on the back nine today", he said. If the course gets more water, it will be hard pressed to play fast and firm and tougher.
Koepka is one of those players that the smart guys like to put in their fantasy league. His best finish was a tie for 37th a year ago at Oakmont. He was 13th previous year at Oakmont. When I get down he kind of tells me a joke or something dumb. You can still make birdies out there.
"Yeah, it's a good spot to be in", he said. He had five finishes in the top 10.
"You play 54 holes around here before the tournament, I felt really really comfortable".
"There are always ups and downs out there".
A do-over chip and two putts later, Hadwin's scorecard had its first double bogey of the championship.
"The only expectation I had was to be low (amateur)", Champ said. This is only his eighth major, and the third time he'll be playing on the weekend.
"I've got a busy summer, so I'm excited to play a lot of golf".
Six players are at 4-under, including Chez Reavie, the other player to shoot 65.