Novak Djokovic was booed off court by some Australian Open fans after retiring injured from his semi-final against Alexander Zverev.

Djokovic, who was bidding for a record-extending 11th title, quit after losing the first set 7-6 (7-5) in 80 minutes.

The 37-year-old Serb had his upper left leg heavily taped after injuring it in Tuesday's quarter-final win over Carlos Alcaraz.

"I did everything I possibly could do to manage the muscle tear that I had," said Djokovic, who was also aiming for an all-time standalone record of 25 major titles.

"Towards the end of that first set I just started feeling more and more pain.

"It was too much to handle for me."

After he slapped a volley into the net on set point, seventh seed Djokovic immediately approached Zverev and shook his hand.

He waved to the crowd and gave them a double thumbs up, despite audible boos from some sections inside Rod Laver Arena.

"I knew even if I won the first set, that it's going to be a huge uphill battle for me to stay physically fit enough to stay with him in the rallies for two, three, four hours," added Djokovic.

"I don't think I had that today in the tank."

German second seed Zverev is still bidding for his first Grand Slam title after twice losing in major finals.

He will face either Italian top seed Jannik Sinner or American 21st seed Ben Shelton in Sunday's final.

From the moment Djokovic lunged for a drop-shot towards the end of the first set against Alcaraz and immediately grimaced to his box, there have been questions over his fitness.

Djokovic, as he has on countless occasions in his stellar career, somehow managed to defy the injury to earn a remarkable win against the 21-year-old.

But doubts remained how equipped he would be to play Zverev.

Djokovic had not practised at Melbourne Park since Tuesday night's victory, cancelling a planned hit on Thursday before spending an hour warming up on court shortly before the semi-final.

Djokovic did not practise between matches during his 2023 title run and later revealed he had a three centimetre tear in his hamstring.

He also won the 2021 title despite tearing an abdominal muscle in the third round.

"The very first thing I want to say is, please guys, don't boo when a player is injured," said Zverev, addressing the crowd in his on-court interview.

"I know everyone paid for tickets and wants to see a five-set match but you have to understand Novak Djokovic is someone who has given absolutely everything to tennis.

"He has won this title with an abdominal tear, he has won this title with a hamstring tear.

"If he cannot continue this match, it means he really cannot continue."

Djokovic looked way below his best from the start of the semi-final.

The former world number one was fortunate not to be punished more by Zverev as he struggled badly with his first serve.

Zverev, playing passively behind the baseline, produced poor errors on the four break points he created in the third game of the match.

Two forehands and a backhand were meekly dumped into the net before he framed a forehand into the front rows of the stand on the fourth.

After three slogs of games spanning 23 minutes, Djokovic had three break points himself at 2-1 but could not take his chances.

Djokovic's service games improved but he had to save another break point at 4-4, and the laboured walks to the chair and anguished facial expressions became more pronounced.

Nevertheless, ending the match early came as a shock to most of the 15,000 crowd on Rod Laver Arena - and Zverev himself.

Asked if he had any indication Djokovic was struggling, Zverev laughed: "No, I actually thought it was a high-level set.

"Of course there were some difficulties and the longer you continue maybe the worse it gets.

"Maybe in the tie-break he was not moving as well, but I thought we had extremely long, physical rallies."