MELBOURNE, Australia (AP) — Spectators booed an injured Novak Djokovic as he left the court in Rod Laver Arena after quitting one set into his Australian Open semifinal against Alexander Zverev on Friday.

Dealing with what he said was a torn muscle, Djokovic lost the opening set 7-6 (5) when he put a forehand volley into the net, then began shaking his head and immediately walked over to tell Zverev the match was over. The 37-year-old Djokovic packed up his equipment and walked off toward the locker room, pausing to respond to the jeers by giving two thumbs-up.

At his news conference, Djokovic said the pain in his taped left leg was “getting worse and worse.” He hurt it during his quarterfinal victory over Carlos Alcaraz on Tuesday night.

“I knew,” Djokovic said, “even if I won the first set, it was going to be a huge uphill battle for me.”

He was bidding for an 11th championship at the Australian Open and record 25th Grand Slam title overall. Instead, it will be No. 1 seed and defending champion Jannik Sinner facing No. 2 Alexander Zverev in Sunday’s final.

Sinner overcame some third-set cramping and beat No. 21 Ben Shelton of the United States 7-6 (2), 6-2, 6-2 in Friday’s second semifinal to return to the Australian Open final as he seeks a third Grand Slam title.

Zverev is 0-2 in major finals; this will be his first at Melbourne Park.

“Everything can happen,” said Sinner, who is on a 20-match unbeaten run. “He’s an incredible player.”

For Djokovic, this is the second time in the past four major tournaments he was unable to finish because of an injury: He withdrew from last year’s French Open before the quarterfinals because he tore the meniscus in his right knee during a match.

Djokovic underwent surgery in Paris and, less than two months later, reached the final at Wimbledon, then won a gold medal for Serbia at the Paris Olympics.

Zverev, a 27-year-old German, lost finals in five sets at the 2020 U.S. Open and 2024 French Open.

“My goal is still to compete with the big guys and to compete for these kind of tournaments and try to win them,” Zverev said. “For that, I need to get better. I need to improve on the court. I need to improve physically.”

During his on-court interview, Zverev pleaded with the fans not to give Djokovic a hard time.

“I know that everybody paid for tickets and everybody wants to see hopefully a great five-set match,” he said. “But you’ve got to understand — Novak Djokovic is somebody that has given this sport, for the past 20 years, absolutely everything of his life.”

The only set of Djokovic vs. Zverev lasted 1 hour, 21 minutes and included 19 points that lasted nine strokes or more apiece. The first four games alone lasted 31 minutes, slowed both by the lengthy baseline exchanges and Djokovic’s deliberate pacing between points, taking the 25-second serve clock down to — and occasionally slightly beyond — the full allotment.

The match was grueling — and would have been even without dealing with a leg problem that initially became an issue late in the first set against Alcaraz.

“I didn’t hit the ball (from after the) Alcaraz match until like an hour before today’s match,” Djokovic said.

“I did everything I possibly can to basically manage the muscle tear that I had. Medications and, I guess, the (tape) and the physio work helped to some extent today,” he added. “But towards the end of that first set, I just started feeling more and more pain and it was too much for me to handle. Unfortunate ending, but I tried.”

Zverev said he could sense “some dents” on the other side of the net in the tiebreaker and noticed that Djokovic was struggling “maybe a bit more.”

Two years ago at Melbourne Park, Djokovic hurt his left hamstring but still managed to depart with the trophy. Against Alcaraz, he was down a set against someone who is 16 years his junior but won.

This time, Djokovic could not pull off a similar escape.

And afterward, there was a lot Djokovic was unsure about.

Might this have been his last appearance at Melbourne Park?

“There is a chance. Who knows?” Djokovic replied. “I’ll just have to see how the season goes. I want to keep going.”

He said it’s too soon to know how long he might be sidelined.

He said he isn’t sure yet what will happen to his coaching arrangement with former on-court rival Andy Murray.

What Djokovic did make clear: His focus and goals will not waver.

“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,” he said. “But I’ll keep going. 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.”

Howard Fendrich has been the AP’s tennis writer since 2002. Find his stories here: More AP tennis: