Novak Djokovic is 37 and was forced to stop playing because of an injury at two of the past four Grand Slam tournaments.

Novak Djokovic has reacted to the crowd at the Australian Open booing after he decided to retire from his semifinal against Alexander Zverev. The Serbian tennis player, 37, had taken to the court with his left leg heavily taped following Tuesday’s injury.

An ailing leg leads to an abrupt finish for the 24-time major winner, who’s a 10-time winner in Melbourne.