He started the day at four over par, the world number four getting off to a promising start with two birdies in the first six holes. Two bogeys and a birdie took him to the 16th hole at three under and a pinpoint tee shot at the par-three there gave McIlroy an excellent birdie chance from inside five feet to relieve the pressure.

Just as his putting had let him down on day one, when five putts inside 10 feet went begging, his Achilles' heel re-emerged at the most crucial of times and he missed the birdie chance. A longer birdie putt went in at 17, the first of the two closing par-fives, giving renewed hope as McIlroy went to the last at two over, right on the projected cutline.

Yet this time it was his iron play and short game that let the Irishman down. Having missed the green with his approach from 210 yards, McIlroy duffed his chip from rough beneath the green and his second attempt only made the collar. That left him needing to hole out from there for par but he failed to deliver and a place in the weekend's play was beyond him at three over par following a one-under par 69.

“I've hit the ball really well this week and I'm walking away not playing the weekend,” McIlroy said. “It's really disheartening. I need to go back to the drawing board and see where we go from here.

“Tee to green is good, I just need to figure out what to do on the greens. I need to have a long hard think about that.”

McIlroy, who's Irish Open victory is his only win of the season so far, has now gone two years without a major victory and he described his 2016 campaign in the majors as “disappointing”.

“Driving the ball well, hitting good iron shots. I think if you had to given anyone else in this field my tee shots this week, they would have been up near the top of the leaderboard. It just shows you how bad I was around the greens.”

Walker and Streb, both of them seeking their first major victory, will lead the field into Saturday's third round at nine under following impressive second rounds. Walker played flawless golf with five birdies moving him to 10 under before a bogey at the 18th left him with a four-under-par 66, pegging him back a shot at nine under.

He was joined there by another Oklahoma-born golfer, Streb, eight years Walker's junior at 29, whose 63 tied the best score in major history, the 31st time it has been carded and matching, amongst other Stenson's closing effort at Troon, and Phil Mickelson's opening round in the same tournament.

For Walker, his ability to contend this week has been a long time coming after five PGA Tour victories in 2014-15 but a winless season so far in 2016.

“I feel close. I can't describe it any way other than that. It's right there. It's just a matter of time before the good play and continuous good play was going to start happening. You kind of got to wait for it. You don't know when it's going to happen. I feel like everything was starting to trend the right way.

“It's cool. It's really cool. You're at the PGA Championship. You are at the top. It's what you are here to do. It's fun.”

The American pair enjoy a two-shot lead over Argentina's Emiliano Grillo, who shot a 67, and Australia's Day, with a 65. The defending champion had double bogeyed the seventh hole then birdied seven of the next eight holes to set up the possibility of a major-record round of 62 with the two closing par-five holes looming but had to settle for pars on each.

A further stroke back on six under lies Open champion Henrik Stenson, whose second 67 of the week puts him in the frame for back to back majors following his success at Troon a fortnight ago.

There is also plenty of quality behind Stenson, with two-time major winner Martin Kaymer and Americans Patrick Reed and Brooks Koepka on five under, Reed having shot a second-round 65.

On a poor day for Irish golfers, only Padraig Harrington made it through to the weekend, his second-round, level-par 70 getting him to one over par as Shane Lowry, Darren Clarke and Graeme McDowell joined McIlroy in making an early departure.