Harman, Fleetwood and Koepka all shot 70’s in the afternoon to join Casey at the top, one ahead of Jamie Lovemark (69), JB Holmes (69) and overnight leader Rickie Fowler, who followed his opening 65 with a 73.

American amateur Cameron Champ, qualifier Xander Schauffele and Brandt Snedeker, Korea’s Si Woo Kim and Japan’s Hideki Matsuyama, who shot a best of the day 65, are two off the pace in a share of eighth on five under.

But for the first time since the inception of the Official World Golf Ranking in 1986, the world’s top three players missed the cut — which fell at one-over par — for the first time in a major.

World No 1 Dustin Johnson looked set to make it when he got back to level par for the championship with six holes to go.

But the defending champion bogeyed the 13th, 14th and 17th to add a 73 to his 75 and miss out by three shots alongside Paul Dunne (73) on four-over-par.

Rory McIlroy missed by four shots despite improving on his first round 78 with a 71, finishing tied for 102nd with Graeme McDowell (73) on five-over par as Jason Day finished well down on 10 over after a 75.

Shane Lowry made it on the mark after a 74 but is now eight shots off the lead and tied for 55th.

Dunne was disappointed to miss the cut on the professional debut in a major after a 73 that featured four birdies and five bogeys.

“The start kind of killed me,” Dunne said of bogeys at his first two holes.

"I hit great shots on 10 and 11. One of them came up short; the other airmailed the green. I don't really know how they went short and long and so I started with two bogeys.

"I hit every shot straight down the fairway, straight down the flag and from there I was kind of chasing it.

"I played okay but every step forward I took a step back. I couldn't get anything going. I had a few just go in the fescue and had to chip out. I didn't play great, but I played better than one over. But that's the way it is."

Asked what he learned from his first US Open, the Greystones man could think of little positive and will now regroup and hope to qualify for The Open by winning one of the spots on offer at Royal Birkdale for the top three non-exempt players finishing in the top ten at the French, Irish and Scottish Opens.

"It is hard to tell now just walking off the course. Another golf tournament and a missed cut," Dunne said. "Probably not a huge amount to be honest.

"I may look back on my preparation and see how that went. I just didn't play well. I didn't really have any parts of my game where I needed them all week. Maybe just play better when I get here.

"I think your touch is more important because greens are quicker. So a chip that would normally stop three feet away is going to roll out to six feet and the same with lag putting.

"I would say your touch would have to be sharper and there is more of a penalty for driving the ball (badly) but that's the way it should be.

"My short game wasn't as sharp as it normally is, for whatever reason I am not sure. My ball striking was very hit and miss so I probably deserved to miss the cut."

McDowell is also chasing one of those spots in The Open and will now take 10 days off to prepare for a long run in Europe.

"I played much better today for sure," McDowell said. "I don't know what happened yesterday.

"A, I wasn't ready for the speed of the greens and B, I don't think I was ready for the type of scoring that we saw.

"Going into this week the golf course was a little bit of an unknown quantity, and I certainly wasn't expecting a plus one cut here.

"I think scoring has been incredibly strong and that speaks to wide fairways, receptive greens.

"But I still think this golf course was tricky. If you weren't 100 percent on your A game, it found you out quite quickly.

"I got off to a slow start yesterday and just mentally never recovered. Today I just made nothing on the greens. Disappointing but it just wasn't there the last two days. We'll regroup and get ready for a nice run in Europe."

While he has falled to 89th in the world, McDowell's missed cut at Erin Hills was just his second failure in 14 starts this year.

"It's tough to miss the cut by four and stand here and tell you I am close, but listen, I saw a lot of good things this week," he said.

"Generally, I like the way my game is moving and trending. This was not a good set up for me, and I just don't think I was prepared to play the kind of golf that was needed to be on the leaderboard this week.

"It's disappointing but there is a long summer of golf ahead, and the great thing about this game is that there's always another chance next week."

Confident he can qualify for The Open, he revealed that he will not enter Local Final Qualifying if he fails to win his place at Royal Birkdale via the French, Irish or Scottish Opens.

"This run of France, Ireland and Scotland is a big pivotal part of my year for me," he said. "I need to get myself in the Open Championship and B, I need some world ranking points for the rest of the year.

"It is a good stretch coming up for me, and I will get back and spend nine days with the family and then jump on a plane next Sunday and potential be gone for five or six weeks. I am looking forward to the summer.

"I will not go to the local qualifier where your chances of getting through are not quite like winning the lottery numbers but it's three or four spots from 100 guys, and that's not the kind of odds I am looking for.

"I'd rather get back to Florida and have five or six days with the family before playing the RBC Canadian Open the week after.

"I believe I can make The Open, very much so. I think I am playing well. I am going to a course in France which I love. And then the Irish Open, I grew up in the area and have played Portstewart many times.

"The Scottish has been good to me in the past as well. So no panic. We will regroup the next ten days and get ready for the French Open."