Some studies says vegan diet all alone cannot sustain longer when your having sports activities, you should mix it with lean meat instead. You can turn to fish if you don't want poultry animals which feed with processed feeds. However, if you're talking about the ability of these athletes I believed Nadal is more energetic and powerful over those mentioned guys.
Nadal is probably physically most dominant tennis player ever, he is a beast so its not even close comparison between him and those two other guys.It is a miracle that he is still so good when you think that he is already 34 years old and how much he uses his body in every match.What Cristiano Ronaldo is in footbal, thats Nadal in tennis.
Djokovic should really give up on that vegan crap and start eating normally.Professional sport is not healthy in general and you can make it even worse if you dont have proper diet.