Fourth seed Daniil Medvedev doused a dangerous threat from Filip Krajinovic Saturday to extend his win streak, battling through buttock pain to stay on track for a best showing at the Australian Open.

The Russian won the first two sets against the Serbian 28th seed but then found himself in a dogfight before prevailing 6-3, 6-3, 4-6, 3-6, 6-0 on a Rod Laver Arena bereft of fans.

He will now play Mackenzie McDonald for a berth in the quarter-finals after the unseeded American cruised past South African Lloyd Harris in straight sets.

"He played unbelievably well, especially the fourth set," said Medvedev, who has never gone beyond the round of 16 at Melbourne Park in four previous attempts.

"He raised his level. I tried to change my position on return in the fifth and seems to have helped. As soon as I got the break, I was always in control."

Medvedev has now gone 17 matches unbeaten, dating back to November and including title wins at the Paris 1000, the ATP Finals in London and the ATP Cup.

He dominated Krajinovic with pin-point serving accuracy and some scintillating tennis from the baseline to take the opening set then broke him in the sixth game of the second.

But the Serb, a training partner of world number one Novak Djokovic while in Adelaide quarantine ahead of the Australian Open, then made Medvedev sweat.

He forced him to save two break points while serving for the second set then broke him for a 3-1 lead in the third.

The Russian stormed straight back but couldn't finish the job, and against the odds Krajinovic pounced to break again and take the set when Medvedev sent a backhand wide.

Medvedev's movement appeared impaired in the fourth set and he took a medical time out when 5-2 down.

"Left buttock," he was heard telling a trainer. "I felt in the second set and it's growing a little bit."

Krajinovic closed out the set, but a revitalised Medvedev broke Krajinovic three times as he rattled through the fifth set without dropping a game in just 25 minutes.

Medvedev is bidding a maiden Grand Slam title and to become only the third Russian man to win a major after Yevgeny Kafelnikov and Marat Safin.