The former world No 3 took two attempts to move his ball out of the hay and even then could only muster a triple bogey from off the putting green.
"I lost a bit of skin out there", Casey said of the triple bogey. Just look at the hole location sheet for the day's play on Friday; nearly all of them are hidden away near the edge of greens.
The second round at Erin Hills was played under sunny skies with comfortable conditions.
"It was a good display - all my own fault - but a good display of what can happen if you get out of position on this golf course, which is what I did on 14", Casey said.
A whopping eight players are at 6 under, including world No. 4 Hideki Matsuyama and Louis Oosthuizen.
The future could be now for Casey who starts the day at seven-under in a four-way tie for the lead alongside compatriot Tommy Fleetwood and Americans Brooks Koepka and Brian Harman. A bogey at the extremely hard par-4 fourth hole was the only blip on the front nine and he made the turn at one-over for the day.
Fowler won the Honda Classic in March, and the 2015 PLAYERS Championship.
Strong to severe storms possible
More storms are expected to develop across SE Kansas and Missouri this evening, and then move our way for late tonight. After getting battered by a storm on Tuesday, the area looks like it could be facing round 2 of storms.
On the golf course, there was only a barrage of birdies.
McIlroy has birdied three of his last four holes.
"You won't get a better day for scoring", Johnson said wistfully during the long walk to sign his card. Moments later, he began a run of five straight birdies that put him right where he wanted to be going into the weekend.
"I really like this golf course; it sets up really well for me, especially if I'm driving it like I did today, " said Johnson, who shot 73 along with a first-round 75 to finish at 148.
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.
"There's nothing I can hide behind; I just have to go out there and get it done". I got off to a good start but it sort of caught up with me as the round went on.
Shane Lowry is the only Irish golfer now inside the projected cut mark of 1-over at the US Open as Day 2 unfolds.
McIlroy, however, who had come into the tournament from an injury layoff after a niggling rib problem, said he was encouraged by his performance. "I think that can't be underestimated, having your friend with you, whether you're leading in the US Open or playing anywhere", he said. I feel like the golf game is there.