Novak Djokovic retired injured after losing the first set 7-6(5) against Alex Zverev on Friday to put the German through to his first Australian Open final.

Zverev will meet the winner of the Jannik Sinner and Ben Shelton match in Sunday's decider as he bids for his first Grand Slam title.

Djokovic shakes hands with Zverev after match. Photo: Reuters

Djokovic's long wait for a record 25th major title will go on, a year after he exited Melbourne Park with a semi-final loss to Sinner.

There were doubts about his fitness ahead of the match and he came out at Rod Laver Arena with his left thigh strapped.

But there was little indication Djokovic was struggling until late in the set when he noticeably began trudging slowly between points and muttering in frustration at his players' box.

Djokovic reacts during match. Photo: Reuters

After losing the set, he promptly informed Zverev that he could not go on and shook hands with the chair umpire before exiting to a chorus of boos from the crowd.

"The very first thing I want to say is, please guys, don't boo a player when he goes out with injury," Zverev said in his on-court interview.

"I know that everybody paid for tickets and wants to see hopefully a five-set match.

"He has won this tournament with an abdominal tear, won this tournament with a hamstring injury.

"So please show some respect."