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).

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.