Novak Djokovic admitted he was struggling physically after surviving a gruelling examination from Daniil Medvedev to reach the Australian Open quarter-finals.

The six-time champion has now reached the last eight at least 10 times at all four grand slams thanks to a 6-4 6-7 (5) 6-2 6-3 victory over his 22-year-old opponent lasting three hours and 15 minutes.

After Stefanos Tsitsipas' win over Roger Federer on Rod Laver Arena on Sunday, Medvedev was trying to strike another blow for the next generation against the old guard.

He did have chances but ultimately could not take them, although it was Djokovic who was angry after losing the second set from 4-1 up.

Asked in his on-court interview at 12.45am how he was feeling, Djokovic said: "Since I guess my next opponent is watching, I'm feeling fantastic, I've never felt fresher in my life."

But in his press conference later, the world number one, who suffered a heavy fall in the third set, admitted that was not the truth, saying: "I didn't feel so great in the last 20 minutes of the match or so.

"So we'll see tomorrow how the body reacts, but I'm confident I can recover and I can be ready for next one. It was not 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 after a match like this."

In the next match, Djokovic will face a man who has spent even more time on court in eighth seed Kei Nishikori, who needed five hours and five minutes to see off Pablo Carreno Busta, coming from two sets down to win 6-7 (8) 4-6 7-6 (4) 6-4 7-6 (8).

It was Nishikori's third five-set match of the tournament, the second time he has come back from two sets down and the second time he has won on a deciding tie-break.

The match had a controversial ending, with Carreno Busta furious that a point when he was leading 8-5 in the tie-break was not replayed after his shot was erroneously called out. The umpire ruled Nishikori had hit a winner before the call came.

Carreno Busta protested vociferously and then lost his cool at the end of the match, swearing and screaming at the umpire as he left the court.

The Spaniard was tearful and full of remorse afterwards, saying: "Obviously I'm very sad because after five hours fighting, the way that I leave from the court wasn't correct, and I'm so sorry, because that's not me.

"I tried to leave as fast as possible when I lost that last point, because I know that in any moment I will lose the head."

Fourth seed Alexander Zverev suffered his latest grand slam disappointment with a bad-tempered loss to Milos Raonic.

The 21-year-old beat Federer and Djokovic back-to-back to win his biggest title at the ATP Finals in November.

But he was unable to improve his record of having reached only one slam quarter-final as he suffered a heavy 6-1 6-1 7-6 (5) defeat to resurgent Canadian Raonic.

After breaking serve in the opening game of the match, Zverev incredibly lost 12 of the next 13 games and showed his frustration by destroying his racket after going 4-1 down in the second set, smashing it repeatedly on the ground.

"I played bad," said Zverev. "The first two sets especially I played horrible."

There was no doubt about Zverev's commitment to smashing his racket, eventually flinging the mangled mess of metal and string dismissively to the court.

"It made me feel better," he said. "I was very angry, so I let my anger out."

In the last eight, Raonic, who began the tournament by beating Nick Kyrgios and Stan Wawrinka, will take on France's Lucas Pouille after he reached his first slam quarter-final since 2016 with a 6-7 (4) 6-4 7-5 7-6 (2) victory over 11th seed Borna Coric.