Rory McIlroy is three shots off the lead as American Alex Smalley took a late grip of the US PGA Championship.

Smalley birdied four of the last six holes to move two shots clear of a packed leaderboard at six under par ahead of what could be a thrilling battle on Sunday.

A five-strong group featuring Jon Rahm, Ludvig Aberg and England’s Aaron Rai are two shots adrift, with a staggering 21 players within four shots of Smalley.

McIlroy shot a four-under superb 66 to surge his way back into contention, having recovered from his disappointing opening 74.

As McIlroy walked off the course, the leading groups were just beginning their final round and he was left hoping they would be unable to stretch the mark too far.

He remains firmly in the mix. As Rahm finished his media duties, he looked at the leaderboard and asked if anyone had seen anything like it.

Much of the talk over the first two days was about how tough a challenge Aronimink Golf Club has presented but it has created a thrilling final-day scenario.

“It’s frustrating to us but, at the same time, it creates a hell of an entertaining championship. If I wasn’t playing this tournament, I’d love what’s going on this week,” McIlroy said.

Rahm added: “As hard as it is to play, the challenge can also be kind of fun if you do well.

“That’s probably the reason why the leaderboard is so bunched up and it’s going to be such a good Sunday tomorrow. So in that sense, showmanship-wise, they’ve done a great job.”

McIlroy was one of a number of morning starters who produced a low score on Saturday, with Justin Rose, Chris Kirk and Kristoffer Reitan all shooting 65 to close within two of the overnight leaders.

The later starters Rai and Rahm briefly nudged the lead to five under but both bogeyed the 18th as they carded excellent 67s.

Overnight co-leader Smalley, in just his fifth major championship, wobbled early with three bogeys in his first four holes but hit back brilliantly.

The American bogeyed 17 to drop back to five under but held his nerve to birdie the last and stand alone at the top.

World number one Scottie Scheffler struggled and carded a one-over 71, which leaves him five back.

“Going into tomorrow, it’s quite literally anybody’s tournament,” Scheffler said. “There’s a lot of guys that have a chance.

“Somebody is going to have a great round and I’m going to make sure to do my best to give myself my the best shot at being the one who has a great round.”

German Matti Schmid and Canadian Nick Taylor matched the lowest round of the week with 65 to sit two off the lead at four under.

Should any players still be tied at the conclusion of the final round, the winner will be decided by a three-hole aggregate score play-off.