Novak Djokovic says he may have played his last Australian Open after the 10-time champion shocked the tennis world by suddenly retiring from his semi-final with a muscle tear.

Djokovic lasted 81 minutes against German Alexander Zverev at Rod Laver Arena on Friday before a gasp rang out when he uncharacteristically butchered a simple volley to lose the opening set in a tiebreaker. But a bigger gasp followed when the 37-year-old announced he could not continue.