First-round leader and World No. 9 Rickie Fowler patched together an even-par 35-37=72 in the final round of the 117th U.S. Open to wrap on 10-under 278, good for a three-way T5, a distant six in arrears of champion Brooks Koepka, who shot 67.
He recovered to drain a 13-foot birdie putt on the par-three 13th, but thereafter was unable to build any momentum over the remainder of the opening nine holes.
In fact, you have to go down to 19 place to find a victor of a major, Sergio Garcia at three-under.
Koepka made three birdies on the back nine, including a handsome putt on 16.
The 28-year-old started day two with a one-shot cushion but a one-over-par 73 opened the door for a four-way tie at the top. They were joined by Brian Harman and Tommy Fleetwood of England, who each had a 70.
It was the second straight year McIlroy missed the cut at the U.S. Open.
Brandt Snedeker, Players Championship victor Kim Si-woo, Xander Schauffele and amateur Cameron Champ are also two shots back at five under. A day after signing for a 63, Justin Thomas (75 for 280) floundered.
Casey made a snowman - a triple-bogey 8 on the par-5 14th - and still broke par with a 71.
"I'm very excited and very happy with the score", the Japanese star said.
The 18th hole at Erin Hills has a special flag for the final round of the U.S. Open.
"Next time I'll probably stay patient and think I will be better".
Thomas bogeyed three of his first five holes to drop back to 8 under.
With a gusty wind blowing across Erin Hills, Spieth's first shot at the par-3 13th hole landed about 5 feet from the flag.
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.
Harman made par on every hole on the front nine except three where he birdied.
He never made the cut at the U.S. Open since turning pro in 2009 until this year.
And Casey was joined at the end of play by compatriot Tommy Fleetwood, and U.S. duo Brooks Koepka and Brian Harman on seven under par.
But Harman is playing the best golf of his career.
McIlroy, the No. 2 player in the world, also arrived last Friday having missed the last month to rest a nagging rib injury.
Dustin Johnson, Rory McIlroy and Jason Day - the top three players in the world, all of them professing expansive Erin Hills to be ideal for their games - spent Friday cleaning out their lockers after missing the cut.
Now the reigning Abu Dhabi HSBC Champion is two rounds shy of earning a whopping $2.1million first prize windfall. Starting on the back nine, he exchanged a birdie on 11 with a bogey on 12. "I just didn't play well". He led by two shots when he finished the third round, but had fallen behind by one by the time the final round began. He says the greens on his last few holes "were firming up".
"I'll take it exactly where I'm at; that was the goal going into the week, to put ourselves in a position to be in contention on Sunday", Fowler said. There no longer is a 10-shot rule that would allow anyone within 10 shots of the lead to make the cut.
The 30-year-old from Georgia had never gone past the second round in a US Open before this year, missing the cut on his two previous visits in 2012 and 2015.