Rory McIlroy conceded runaway leader Brooks Koepka was playing golf on a "different level" as his major drought continued in the US PGA Championship.

McIlroy's second US PGA title at Valhalla in 2014 remains his last to date and the 30-year-old was never in contention at Bethpage after opening rounds of 72 and 71 on Long Island.

The former world number one had even looked set for an early exit after dropping five shots in his first three holes on Friday to slump to seven over par, but played his last six holes in four under to make the cut with a shot to spare.

But although he started round three with a front nine of 32, McIlroy had to settle for a 69 to finish two over par and 14 shots behind Koepka, whose halfway total of 128 is the best in major history and gave him a tournament-record seven-shot lead over Jordan Spieth and Adam Scott.

Koepka is chasing a fourth major title in his last eight starts and McIlroy said: "It's awesome. It's so good. It's great to watch and I watched most of it yesterday afternoon.

"He's definitely, in these events, playing on a different level than most anyone else. It will be fun this afternoon to see if he goes out there and just keeps his foot down and extends that lead.

"It's firming up a little bit but I still think it's pretty scoreable. If Brooks hits it the way he's hit it the first couple of days I can see him shooting a good score in the 60s and if he does that obviously it's going to be very hard to get near him."

Rory McIlroy will have to wait for a third PGA Championship win

McIlroy missed good chances for birdie on the first three holes, but made amends by holing from 30 feet for an eagle on the par-five fourth before coming up inches short of a hole-in-one on the eighth following a brilliant five-iron approach from 224 yards.

However, the Down man was then unable to get up and down from a greenside bunker on the difficult 10th and three-putted the 14th, where his par putt caught the edge of the hole and span out.

"It definitely could have been better, it's the best I hit it all week for sure," McIlroy added. "I sort of got it going on the front nine there, (hit) a couple of loose shots on the back nine but I played well. I didn't convert as many chances as I would have liked but it was an improvement on the last couple of days.

"I hit the ball as good as I've hit it in a while, gave myself plenty of chances. If I had made a couple of putts I could definitely have shot 65, 66 easily out there today.

"If you look at the last nine I played yesterday and the first nine today I was seven under par for those 18 holes so it's just about putting it all together on the right day.

"I definitely felt like I left four or five out there, but you have to put it all together to shoot a really good score. I was close to doing that today but just not quite."

Joining McIlroy as the leading Irish player on two over par is Shane Lowry, whose 68 remains the low round of the day in the clubhouse alongside Thailand's Kiradech Aphibarnrat as the leading pairs begin to tee off at the New York venue. 

Lowry, who finished tied-12th in Bellerive last year, has only carded two bogeys across Friday and Saturday combined and birdies this morning on the 2nd, 11th and 14th saw him complete an impressive round of 68.

"Playing golf on the cut mark is quite difficult sometimes. I was able to go out and play a bit freer today and kind of let it go a little bit," Lowry said afterwards.

"I drove it well the last couple of days and I feel like I missed a couple of chances coming in as well. But anything under par around here is good.

"I don't know what happened the first day. I think it might be an element of paying the golf course a little bit too much respect."

Graeme McDowell endured a less satisfactory third round, the 2010 US Open champion caring a three over par round of 73 to fall back to five over par for the championship.