MELBOURNE, Australia -- Twenty-four-time Grand Slam champion Novak Djokovic retired from his Australian Open semifinal match against Alexander Zverev on Friday afternoon, unable to continue because of a muscle tear in his left leg.

After Zverev had secured a marathon 81-minute first set in a tiebreak, Djokovic, who was playing with heavy strapping on his left thigh, walked gingerly to the net and shook hands with the world No. 2, conceding the match and confirming Zverev's place in Sunday's final.

The capacity crowd at Rod Laver Arena booed Djokovic as he headed for the exit, and he responded by flashing two thumbs-up.

"I did everything I possibly can to manage the muscle tear that I had," a dejected Djokovic told reporters minutes after stepping off the court. "Medications and the strap and the physio work helped to some extent today, [but] towards the end of that first set I just started feeling more and more pain. It was getting worse and worse. It was just too much to handle for me at the moment.

"I knew even if I won the first set it was going to be a huge uphill battle for me to stay physically fit enough to stay with him in the rallies for another, god knows, two, three, four hours. I don't think I had that, unfortunately, today in the tank. Unfortunate ending, but I tried."

Zverev, 27, who will play in his first Australian Open final and face defending champion Jannik Sinner, addressed the booing fans and defended Djokovic in his on-court interview.

"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 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."

Djokovic, 37, who was seeking a record-extending 11th title at Melbourne Park, suffered the injury during his quarterfinal win against world No. 3 Carlos Alcaraz earlier in the week.

Friday's retirement marks the second time in the past 12 months Djokovic has been forced out of a Grand Slam through injury. At last year's French Open he withdrew from his quarterfinal against Casper Ruud with a knee issue.

"It is true that, you know, getting injured quite a bit the last few years. I don't know what exactly is the reason for that," Djokovic said. "It's not like I'm worrying approaching every Grand Slam now whether I'm going to get injured or not, but statistics are against me, in a way, in the last couple of years.

"I'll keep striving to win more Slams and as long as I feel that I want to put up with all of this, I'll be around."

Djokovic will now have the injury investigated further to determine how long he will be sidelined.

"When I go back home to Europe I will get together with the medical team and my physios and try to understand what we can do and the quickest way to recover and get back on track," he said. "I still have Doha tournament in a few weeks' time that is scheduled. Whether I'm going to play that or not, it really does depend on how quickly I recover. It just depends on the muscle and how it responds to the treatment."