MADRID -- Novak Djokovic, winner of a record 24 Grand Slam titles, says he can feel the changes taking place in tennis.

People have started to accept, he says, that there is no more Roger Federer, no more Rafael Nadal and — soon — no more Novak Djokovic.

Novak Djokovic's frustrations got the better of him in the build-up to the Madrid Open as he prepares to face Matteo Arnaldi ...