On another day of low-scoring at the Erin Hills course in Wisconsin, Casey and compatriot Tommy Fleetwood were bunched on seven under alongside Americans Brooks Koepka and Brian Harman after the second round.
Behind the leading quartet, a logjam grouped eight players within two shots of the lead.
Jamie Lovemark, Rickie Fowler and J.B. Holmes featured in a three-way tie at six under.
South Korea s Kim Si-Woo and Japan s Hideki Matsuyama were among five players on five under which also included amateur Cameron Champ.
Matsuyama, who had struggled in the opening round with a two-over-par 74, produced an electrifying second round seven-under-par 65 to thrust himself firmly into contention.
American Chez Reavie also scored 65 on a day when Erin Hills failed for the most part to bare its fangs.
With the past six majors going to players who have never won a major before, there is a good chance that run could be extended this weekend.
None of the top 18 players on the leaderboard have won a major championship before and stars such as world number two Rory McIlroy and Australia s Jason Day both missed the cut.
Casey took his place amongst the leading group after recovering from an early implosion that sent him plunging down the leaderboard.
Casey, who had shot a six-under-par 66 on Thursday, came unstuck with a disastrous triple bogey eight on the 14th after teeing off on the 10th hole.
However the 39-year-old world number 14 unfurled a sequence of five straight birdies to haul himself back up the leaderboard, carding a one-under-par 71 to finish the day at seven under.
“Not every day you enjoy a round of golf with an eight on the card, but I m a pretty happy man,” Casey said. “It was a bit of a roller coaster.”
But while Casey was relieved after his gutsy fightback, two of the world s top three players — McIlroy and Day — were left contemplating a missed cut.
World number three Day, needing a miracle to get back into contention after his opening 79 carded a three-over-par 75, leaving him 10 over and way outside the projected cutline.
“I was in the hay too much over the last couple of days,” Day said.
“It s been the best preparation going into a major, I felt like, in my career, which is playing in majors and knowing what I needed to do to prep for a tournament such as this,” Day said. “It s pretty frustrating.”
McIlroy meanwhile — who earlier in the week had advised players unable to hit the fairways to “pack their bags” — was left at five over par after a second-round 71.
Despite a flurry of late birdies it was not enough to turnaround an erratic round littered with errors and missed opportunities.
McIlroy however, who is returning from a hip and back injury, put a brave face on his second consecutive missed cut at a US Open following his exit at the halfway stage in 2016.
“I m optimistic with where my game is,” he said. “Hopefully I ve got a lot of the bad stuff out of my system yesterday and some parts today. It s just a matter of getting competitive rounds under my belt.
“I ve got a busy summer, so I m excited to play a lot of golf. I feel like that s going to help me to get back into contention and hopefully try to win some of these things.”