Novak Djokovic throws his ailing 37-year-old body into an Australian Open semi-final against Alexander Zverev on Friday while defending champion Jannik Sinner faces Ben Shelton. Djokovic and German world number two Zverev are first up at 2:30 pm (0330 GMT) on Rod Laver Arena as the Serbian great closes in on a record 25th Grand Slam title. Djokovic rolled back the years to defeat Carlos Alcaraz, aged 21, in four sets in the quarter-finals in Melbourne. But his upper left leg was taped after a medical timeout and he needed painkillers to battle through.