Watch: Novak Djokovic medically retires from Australian Open semifinal

In his quarterfinal victory over Carlos Alcaraz on Wednesday, Djokovic appeared to fight through an injury he sustained mid-match. It's not clear if the same injury forced his retirement Friday.

This is the first instance of Djokovic retiring from a match after the first set in his illustrious career.

He was seen lifting a thumbs up to the crowd despite some boos from those in attendance at Rod Laver Arena in Melbourne.

Zverev now advances to his third major final, his first at this tournament, despite the unfortunate circumstances. He awaits the winner of the second semifinal between World No. 1 Jannik Sinner and American upstart, No. 20 Ben Shelton.

Zverev spoke to the crowd after his short victory, imploring them to respect Djokovic's decision and support his recovery.

“Please guys don’t boo a player when he goes out with injury," Zverev said. "I know everyone paid for tickets ... but Novak has given everything of his life to the sport the last 20 years."

Zverev has never won a grand slam tournament in his career but he may now have the opportunity to end that drought on Sunday.