"The golf course has got plenty of moisture in it", Justin Rose said after his 72.
Among the early starters on another steamy day at Erin Hills, Casey survived a hair-raising round that included a gut-wrenching triple-bogey and a thrilling run of five straight birdies and ended with him signing for one-under 71. "I told my caddy after we made that bogey [at the par-3 13th] we had some scoring opportunities coming up".
Koepka, Harman and Fleetwood all shot matching two-under 70s to join Casey at seven under.
Overnight leader Rickie Fowler went 28 holes in a row without a bogey and then had three in a row, and this has really made the tournament very interesting. Fowler shot a 73 and was still very much in the hunt at a second straight major.
Oosthuizen, whose sole major title to date came at St Andrews in 2010, birdied the first, fourth, seventh and eighth to move to four under par, while 2015 victor Spieth had covered his first 11 holes in level par. Two shots might separate the top 12 players - with just eight shots spread across the entire field - but the leaderboard is devoid of high-profile players and marquee names. Only a half-dozen of them have even experienced Sunday contention in golf's biggest events.
"Those were tough shots", Fleetwood said. And at this U.S. Open, he showed how a quick recovery is never too far away.
The question is whether Erin Hills delivered either in the opening round. The USGA is providing complimentary bottled water the rest of the week. Johnson birdied his opening two holes of the second round in his bid to first make sure he had a tee time Saturday, and then try to make up ground on the lead. Justin Thomas overcame a rumbling stomach to take a bite out of US Open history yesterday with a record-breaking 63. Harman made a 30-foot birdie putt on the final hole to hold off Dustin Johnson and Pat Perez by a stroke.
Koepka also reached the green in two on the 667-yard closing hole for a two-putt birdie and a 68.
With the entire field separated by just eight shots after the cut fell at one over, 2015 victor Jordan Spieth had hoped to get himself into contention, but struggled to a 76. "But when you're visually looking down the fairway, I think when you're trying to aim at a target at a normal golf course, with normal width fairways, there's some pressure into hitting the fairway because it is a lot narrower than we have out here".
"I hope the moment will come as soon as possible. I just happened to be in the flawless spot".
Despite their respective struggles, neither Johnson, McIlroy or Day took issue with the course, instead taking ownership of the problems that cut their weekend short. But it was too little too late for the Northern Irishman who remains anchored near the bottom of the leaderboard at five over.
McIlroy was never in the short grass from the 11th hole to the end on Thursday when he opened with a career-worst 78, and he didn't do almost enough in the second round until it was far too late.
"It was definitely conducive for good scores today", Thomas said. "Two yards either way, I would have been stuck".
Incredibly, Thomas' round contained two bogeys.
"It's not every day you enjoy a round of golf with an eight on the card, but I'm a pretty happy man". "Yeah, it was a bit of a roller coaster".