MELBOURNE - Novak Djokovic retired injured after losing the first set 7-6(5) against Alexander Zverev on Friday to put the German through to his first Australian Open final and extend his wait for a record 25th Grand Slam title.

Zverev will meet either Jannik Sinner or Ben Shelton in Sunday's decider as he bids for his first Grand Slam title, but Djokovic will remain locked with Margaret Court on 24 major trophies until at least the French Open.

There were doubts about Djokovic's left thigh following his taxing quarterfinal win over Carlos Alcaraz when he took a lengthy medical timeout after the first set against the Spaniard.

The 37-year-old Serb skipped a training session on the eve of the semifinal and entered Rod Laver Arena for the afternoon match with the thigh strapped.

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

He gave up the set by hammering a regulation forehand volley into the net and promptly walked to Zverev's side to shake hands and embrace the German, leaving the crowd gasping.

With a rueful expression, Djokovic held two thumbs up and clapped at the terraces but there was a round of boos as the 10-times champion made his exit.

Djokovic later said he had been trying to nurse a torn muscle and he was in increasing pain as the set wore on.

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"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, you know, for another God knows what, two, three, four hours," the Serb told reporters.

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

'Show some respect'

Zverev said he was also surprised by Djokovic's retirement but noted the Serb's movement had fallen away in the tiebreak.

"I thought it was a high level first set," he added.

"Of course there are some difficulties ... in the tiebreak he was not moving. I did see him struggle a bit more."

Zverev also admonished sections of the crowd for jeering Djokovic, who has dominated the year's opening Grand Slam since winning his first trophy at Melbourne Park in 2008.

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

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

"But ... Novak Djokovic is someone who has given this sport for the past 20 years absolutely everything of his life.

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

"So please show some respect."

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The truncated semifinal win is a huge boost for Zverev's hopes of finally winning a Grand Slam, having missed out to Alcaraz in last year's French Open final and surrendering a two-set lead before losing to Dominic Thiem in the 2020 US Open decider.

The rangy 27-year-old has been tipped for Grand Slam success since his teen years but was often thwarted by the likes of Djokovic and the now-retired Roger Federer and Rafa Nadal.

Zverev was strong on serve on Friday and appeared content to wage attritional rallies with the Serb while biding his time for chances.

Though failing to convert all five break points he had over Djokovic, Zverev was rock-solid in the tiebreak.

He thrashed a forehand down the line to bring up set point with a 24th winner before Djokovic made his stunning exit, a year after being shocked by Sinner in the semifinals.