The 31-year-old Serb earned a 6-4 6-7 (5-7) 6-2 6-3 victory over the 15th seed in Melbourne.

Djokovic was not at his fluent best, but outlasted Medvedev, 22, to win in three hours and 15 minutes.

"A couple of things have surfaced after a match like this," he said.

"I'll see how the body reacts."

Medvedev also struggled physically and grew frustrated as he paid the price for only taking two of nine break points.

However, his resistance continued until the end, saving two match points before Djokovic took the third with a wonderful cross-court winner.

Six-time champion Djokovic will play Nishikori in the last eight after the eighth seed beat Spain's Pablo Carreno Busta in controversial circumstances in a five-hour match decided by a super tie-break.

Top seed Djokovic, who tumbled over in the third set and fell again in the fourth, looked uncomfortable in the final stages of the match.

"I didn't feel so great in the final 20 minutes or so," he said.

"A bit of fatigue, a bit of hurt in the back, but nothing major. I'm confident I can be ready."

Another triumph for the top seed at Melbourne Park will see him move ahead of Australian great Roy Emerson and Roger Federer, who was knocked out by Greek youngster Stefanos Tsitsipas on Sunday, in terms of all-time victories.

Djokovic outlasts Medvedev in physical encounter

Djokovic's progress to the last 16 had been relatively serene, apart from a third-set blip against the run of play in his previous win over Canadian teenager Denis Shapovalov.

But the powerful Medvedev, one of the sport's rising stars and in good form having reached the final of the recent Brisbane International, was expected to provide a sterner test.

It was a test of durability as the pair pushed each other in some long, physical rallies - including two which lasted more than 40 shots.

In stuffy conditions on Rod Laver Arena, both players struggled physically at times with Medvedev appearing to tire first in the third set.

Although there was a suspicion Medvedev was putting up little fight to preserve energy for a push in the fourth, it proved to be the key to Djokovic's victory as he maintained the momentum that had swung his way.

"It was definitely a physical battle," Djokovic said.

"It is difficult to go through him and I had to find another way to go through him.

"In this kind of match you just have to hang in there and wait for your opportunities."

Although Djokovic needed his survival instincts from early in the match, he was also guilty of squandering his own chances in the second set.

After spurning five break points in Medvedev's first service game of the second set, Djokovic finally took one in the next game and then had another for a 5-1 lead which would have left him serving for the set.

Medvedev responded by thumping two aces on his way to holding and Djokovic, whose mood soured as he threatened to damage his racquet, appeared to be rattled.

Yet after Medvedev controlled the second-set tie-break to level, Djokovic reasserted his authority to reach his 10th quarter-final in Melbourne and become only the second man, after Federer, to reach 10 at all four Grand Slams.