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.

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.

Alexander Zverev chided the crowd after they booed Novak Djokovic for retiring due to injury after the first set in an Australian Open semifinal.