After day one, it was Rickie Fowler who sat at the top with the outright lead.
T5. J.B. Holmes (-6): For every bogey on the day, Holmes had an answer with a birdie on the subsequent hole.
American Chez Reavie also scored 65 on a day when Erin Hills failed for the most part to bare its fangs.
"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", he said.
Among the early starters on another steamy day at Erin Hills in Hartford, Wisconsin, 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. With about two hours left in the second round, the cut was looking to be 1-over par, which would tie a U.S. Open record.
Rickie Fowler (-6) - Fowler quickly climbed to 9 under, and looked like he might really make life hard for the rest of the field, but three straight bogeys on his back nine derailed all momentum.
McIlroy was 4 shots off the cut line and Day a whopping 9 shots removed.
Unfortunately for McIlroy, he might well be doing just that on Friday afternoon, the 28-year-old following an eagle on the second - where he drove the green on the 330-yard par four - with four bogeys and double bogeys on the 15th and 17th.
Washington County Sheriff Dale Schmidt told the United States Golf Association he appeared to die of natural causes. "I have no idea how the weekend is going to go, no one does".More news: Too hot to handle: Study shows Earth's killer heat worsens
That victory was redemption after 2015 when, on the bumpy, much-criticized greens at Chambers Bay, he three-putted from 8 feet on the final hole to give away the win, then the tie.
He has birdied three of his last four holes.
Casey, who had shot a six-under-par 66 on Thursday, had threatened to mount an early assault on the leaderboard but came unstuck with a disastrous triple bogey eight on the 14th.
Fowler wasted no time in exploiting the conditions after teeing off on the 10th, rattling off four quick birdies to be four under.
He then hacked out his fifth out sideways and now some 52-feet from the hole from where he chipped and two putted. Sergio Garcia, of Spain, hits a drive on the 12th hole during the second round of the U.S. Open golf tournament Friday, June 16, 2017, at Erin Hills in Erin, Wis. Johnson won by one, though maybe Fleetwood's most memorable moment there, for better or worse, was the embarrassingly bad high-five he shared with his caddie after draining a long birdie putt to pull within one. PGA champion Jimmy Walker missed the cut as he continues to recover from Lyme Disease.
Now consider that the largest 36-hole comeback in U.S. Open history was 11 shots by Lou Graham in 1975. "I'd love to have another one of those", Thomas said. Can he become the seventh straight first-time major victor dating to Jason Day's victory at the 2015 PGA Championship?
However, plenty of others were firmly in contention at the start of the penultimate day; only eight shots separated the leaders from those who made the cut on the bubble.