Novak Djokovic learns 'big lesson' from US Open default

Novak Djokovic learned "a big lesson" after being disqualified from the US Open for unintentionally hitting a line judge in the throat with a ball. The incident eight days ago marked a stunning end to Djokovic's 29-match winning streak and his bid for an 18th Grand Slam title. "I'm working mentally and emotionally as hard as I am...