Spectators booed as the 37-year-old Djokovic walked off toward the locker room, and he stopped, turned around and responded by giving two thumbs-up.

Novak Djokovic is out of the Australian Open in stunning fashion. The 24-time slam champion medically retired immediately after dropping the first set of his semifinal match against No. 2 Alexander Zverev in a tiebreak 7-6 (5).

For Djokovic, the withdrawal means that he won't win his record 25th major title and 11th Australian Open. Injuries have prevented him from finishing two of his past four Grand Slam tournaments. He withdrew from last year's French Open before the quarterfinals due to a torn meniscus in his right knee.