The world No.1 admits he didn't feel too flash in the closing stages of his hard-fought 6-4 6-7 (5-7) 6-2 6-3 fourth-round victory over Daniil Medvedev, a three-and-a-quarter-hour slugfest that finished after midnight on Monday.

Djokovic took a tumble during the third set and was clearly running on empty before pulling the match out of the fire against the rising Russian.

"I didn't feel so great in the last 20 minutes of the match or so," Djokovic said.

"So we'll see tomorrow how the body reacts, but I'm confident I can recover and I can be ready for the next one.

"It wasn't the fall. It wasn't particularly the fall. It was just a little bit of fatigue, a little bit of back. Nothing major.

"But there are a couple of things that have surfaced, so to say, after a match like this."

Djokovic even made the rare confession that he was "hoping the other guy won" after learning Nishikori was his quarter-final opponent.

The Asian superstar upset Djokovic in the 2014 US Open semi-finals, but the Serb seemingly hasn't forgotten that defeat, having beaten Nishikori 11 straight times since.

Nishikori is unlikely to be feeling any fresher than Djokovic for their latest showdown, having spent almost 14 hours on court this campaign just to reach the last eight for a fourth time in Melbourne.

The world No.9 has survived three sapping five-setters and had to battle back from two sets and a service break in the fourth round to escape the clutches of Spaniard Pablo Carreno Busta.

"He's a fighter. He's a very talented player, one of the quickest players on the tour, a hard worker," Djokovic said of Nishikori.

"I have lots of respect for him. I expect a tough one."

Chasing a record seventh Open crown, Djokovic, the reigning Wimbledon and US Open champion, will be contesting his first quarter-final at Melbourne Park since 2016, having lost in the fourth round last year and second round in 2017.

The victory will play the winner of Wednesday's other quarter-final between Milos Raonic and Lucas Pouille for a place in Sunday's title match.

Australian Associated Press