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.

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 was forced to pull out of the Australian Open semi-finals following an injury, bringing further attention to the tennis legend's stance on retirement