And although he birdied the next two and came agonisingly close to making it a hat-trick on the ninth, his final hole of the day, the 28-year-old's second successive 72 left him two over par.

American Kevin Kisner had set an imposing halfway target at eight under par after his second straight 67, with Rickie Fowler his nearest challenger in the clubhouse on three under.

Open champion Jordan Spieth, who needs a third win in four starts to become the youngest player to complete a career grand slam, was two over par after nine holes of his round.

McIlroy, whose only major victory when trailing after 36 holes came when he was two behind in the 2012 US PGA, said: "I probably didn't quite hit it as well off the tee as I did yesterday and wasn't putting myself in positions where I could go at a lot of pins.

"I had a little bit of a mini-rally at the end which I needed but the four bogeys on the front nine, that's just down to being out of position off the tee and not being able to get it very close with my second shot.

"Hopefully (I can) tidy that up going into tomorrow, but obviously Kis is on fire right now. But take him out of the equation, I feel like I'm still right there in the tournament."

McIlroy set the course record at Quail Hollow with a 61 on his way to a seven-shot victory in 2015, but changes to a number of holes and a different grass being planted have made a pronounced difference.

"I guess a low round used to be a 61 or a 62," he added. "A low round now is a 66 or a 67. You're playing your ass off to get that. I'd say if I shoot two 67s over the weekend, I'm going to have a really good chance."

McIlroy admitted his situation could easily have been a lot worse, especially after he carved his second shot on the 591-yard 10th over the crowd and saw it bound down a cart path which runs along the side of the 11th.

The ball eventually came to rest just off the path and left the world number four with a seemingly impossible third shot from around 60 yards, but he produced a brilliant low pitch which bounced off the path, through a bunker and ran across the green before stopping on the fringe.

From there McIlroy chipped to two feet and holed for par, giving a wry smile and raising his eyebrows to members of the media as he made his way to the 11th tee.

"It was really the only option I had," McIlroy explained. "I dropped it in a pretty bad lie, so I couldn't carry it over the cart path. So I hit a six iron and just said, 'okay, I'll bounce it up the cart path and see where this goes'.

"Luckily it got out of the bunker and on to the other side. That could have been a six or a seven quite easily so to get away with a five, I actually felt pretty good about myself going to the next tee."

A birdie on the 15th helped McIlroy to the turn in 35, but he then bogeyed the second, third, fifth and sixth to lie outside the projected cut mark, before birdies on the seventh and eighth saw him safely through to the weekend.

Kisner's best finish in his previous 11 majors is a tie for 12th in the 2015 US Open, but the 33-year-old from South Carolina made the most of his local knowledge to card three birdies, an eagle and a solitary bogey.

"I've been upset with how I've played in the majors so far in my career," Kisner said. "I feel like I have the game to compete and (have had) tons of 30th to 40th, 50th-place finishes.

"This is probably the easiest one I've had to prep for because I know the golf course so well and I've been up here a lot.

"I've spent every Thanksgiving and Christmas in Charlotte from childhood to marriage, got a lot of ties to Quail Hollow. My brother-in-law's father is a founding member here."

Kisner's 67 was eclipsed as the lowest score of the week so far by New Zealand's Ryan Fox, who raced to the turn in 31 and eventually added a 66 to his opening 75 to finish one under.

At the other end of the leaderboard, Phil Mickelson finished 11 over par on his 100th major appearance to miss just his second halfway cut in 25 US PGA starts.

And defending champion Jimmy Walker also bowed out despite a 69 which was a 12-shot improvement on his opening 81.

Spieth bogeyed the 10th to fall even further off the pace before play was suspended at 4:43pm local time (9.43pm Irish Time) due to the threat of lightning.

Kisner's lead had been cut to two shots following a hat-trick of birdies by WGC-Bridgestone Invitational winner Hideki Matsuyama, with former world number one Jason Day a shot further back.