Perhaps making it a bit easier is the knowledge that Erin Hills is nowhere near the suffocating sauna that typically affronts players in this tournament.
And then there is Rickie Fowler, also one back.
"It doesn't need to be too much of a change for this golf course to play pretty hard", said Casey. With about two hours left in the second round, the cut was looking to be 1-over par, which would tie a US Open record.
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With the top 60 and ties qualifying for the final rounds, the cut appeared set to fall at one or two over par.
England's Paul Casey is a shot further back on 7-under after carding a 71 to follow up yesterday's 66.
Paul Casey, Brooks Koepka, Brian Harman and Tommy Fleetwood share the 36-hole lead at Erin Hills. The Englishman shot a one-under 71 but had to fight back from two bogeys and a triple bogey in his first six holes.
"If you look at the golf course and talk to me, Jason or Rory, this course sets up flawless for us", world No.1 Johnson said.
"It's very hard to get get through a major championship, let alone a US Open without a blemish on your card", the 39-year-old told BBC Sport.More news: Kim Kardashian Officially Announces Her Own Makeup Line
A three-time victor on the PGA Tour this season, Thomas also joined the exclusive "59" club by breaking 60 at the Sony Open, but he may have saved his best for Father's Day weekend and his quest for his first major crown. When it was clear he had nothing going in the second round, he was simply trying to stay out of the way. He shot 75 and missed the cut.
Jason Day's tournament was in his words, "frustrating".
Playing just his seventh tournament of the year due to a rib injury, McIlroy left himself a mountain to climb following a first round of 78 on Thursday, when a record 44 players broke par.
McIlroy dismissed the notion of being rusty before the tournament, but conceded on his way out of Wisconsin that he hasn't played almost enough.
The 28-year-old, who is a four-time major champion and won the US Open in 2011, was disappointed to make the cut but pleased to have finally had some competitive action.
Matsuyama vaulted into contention at the U.S. Open with a 7-under 65 on Friday, moving to 5 under and a tie for eighth - just two shots back of the four-way tie atop the crowded leaderboard.
Hoffman birdied five of his first eight holes to get to 9 under, one shot clear of a massive logjam in second.
Another warm, dry day is drying out the course, and the players are noticing a difference.
"A little disappointing to end with the double on 17 but I started hitting it a lot better on the back side", Hadwin said minutes after his round. "But I think definitely getting a lot speedier, a lot faster".
"It's all good holing the putts on the practice green, but it's a different kettle of fish when you get out on the course and have to read slope, read downhill, uphill into the green, side green". "Guys turn up, girls turn up, everyone just does their job they want to be part of something that's fantastic", said Evans.