Tennis is a sport and by being a sport you need to be strong physically to be able to succeed in it but the mental part of the game is even more important.
It's quite easy to prove because so many tennis players have shown in the past that they are not able to handle the pressure of a situation. Players choke, lose matches they should have won, a miss easy shots all because they are unable to handle the moment.
Novak Djokovic is one of the greatest of all time because he's able to handle those moments.
John McEnroe agrees as he thinks the Serbian is the strongest player mentally to ever play the game. He said:
"I respect him even more that he has overcome these things. He has to be the most mentally tough player I’ve ever seen on a tennis court. Nadal is the greatest effort player, but Djokovic is mentally unbelievable. What he is able to deal with and still to succeed at this extraordinary level.”
McEnroe also hopes to see Djokovic playing in New York although he understands that it's unlikely. He blatantly called it a terrible decision.