Novak Djokovic's bid to make more tennis history is still alive as he scraped into the Australian Open semi-finals after Lorenzo Musetti retired injured when leading by two sets.

Djokovic, aiming for a standalone record 25th Grand Slam singles title, was wayward, error-strewn and irritable against the inspired Italian in front of a stunned crowd.

Musetti was 6-4 6-3 up and looked set for the semi-finals when he pulled up early in the third set.

The fifth seed took a medical timeout for treatment on his thigh but, unable to serve or move properly, he walked to the net and shook hands at 3-1 down in the third.

A heartbroken Musetti was helped down the corridor by a member of his team as he left the court.

"He was a far better player - I was on my way home tonight," the 38-year-old Serb great said.

"I don't know what to say except that I feel really sorry for him.

"I really wish him a speedy recovery. He should have been a winner today, there's no doubt."

Djokovic received treatment for blisters on his feet at the end of the second set and could also have been tested physically had the match continued.

It is another stroke of luck for Djokovic after fourth-round opponent Jakub Mensik withdrew before their match with injury.

Djokovic has not won a set since his third-round victory over Botic van de Zandschulp on 24 January.

For Djokovic to move ahead of Margaret Court's record of 24 major singles titles, he will need to limit his time on court, keep the points short and hope some good fortune goes his way.

He has managed those so far - but he would not have anticipated reaching the semi-finals with such a heavy reliance on fortuity.

Djokovic started sharply against Musetti, going an immediate break up, but then spectacularly lost his way.

A dreadful miss on an overhead handed the break back to Musetti and Djokovic never recovered, sending down just one ace and committing 18 unforced errors to nine winners on his way to losing the first set.

Djokovic frequently turned to his box in disbelief and annoyance - but he only had himself to blame at the start of the second set.

After breaking Musetti and getting out to a 30-0 lead on serve, Djokovic hit five unforced errors in six points to hand the initiative back.

Anchored by a one-handed backhand that kept Djokovic guessing and an improved serve, Musetti held off his opponent to move within touching distance of a first Australian Open semi-final.

Musetti's level just made the injury all the more surprising. After being broken in the third set, he immediately called for the trainer and, after a valiant attempt at his next service game, called it quits.

"I honestly have no words to describe how I'm feeling right now," Musetti said.

"I felt it at the beginning of the second set. I was feeling that the pain was increasing, and the problem was not getting away."

Djokovic had warm words for his opponent at the net and afterwards - and, having struggled with physical issues early in his career, would know how devastated the 23-year-old was.

Despite the good fortune, Djokovic will know he will need to improve if he is to win an 11th Australian Open title, with two-time defending champion Jannik Sinner potentially waiting in the last four.

World number two Sinner, who beat Djokovic in the semi-finals two years ago, faces American eighth seed Ben Shelton later on Wednesday.