World number one Dustin Johnson led an unprecedented exodus of big guns at the 117th US Open on Friday as four players grabbed a share of the lead at the halfway stage.

American favourite Johnson crashed out after he could only card a one-over-par 73, which left him four over for the tournament at the sprawling Erin Hills course in rural Wisconsin.

Johnson's exit came after world number two Rory McIlroy and world number three Jason Day also missed the cut after disappointing rounds.

It was the first time since the rankings system was launched in April 1989 that the world's top three players have all failed to make the third round of a major.

Johnson, who had bullishly expected to do well on a course he believed to be perfectly suited to his game, admitted he was surprised to be out after only 36 holes.

"I felt this is a golf course sets up really well for me, especially if I'm driving it like I did today," Johnson said. 

"If you look at the golf course and you even talk to me, Jason or Rory, this course sets up perfect for us."

With Johnson, McIlroy and Day all missing from the weekend action, all eyes will be on a clustered leaderboard which is dominated by players who have never won a major.

England's Paul Casey and compatriot Tommy Fleetwood were bunched on seven under alongside Americans Brooks Koepka and Brian Harman at the top.

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.

"I'm very excited and very happy with the score," the Japanese star said. "There were a couple of loose swings out there, but when you shoot 65, you can take those."

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.

- Casey fightback -

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, it was back t the drawing board for McIlroy and Day after their disappointing exits.

Day, needing a miracle to get back into contention after his opening 79 carded a three-over-par 75, leaving him 10 over.

"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."

SourceL AFP