Nadal's tough, never-say-die, gritty, warrior persona on the court was absolutely revolutionary in tennis. Yes, there had been intense, hard-working, gritty players before, but none ever had the almost cinematic dimension that Nadal had. The funny thing is that he's not even really like that anymore. Which is fine. He's grown up. Things change. But the legend will live on forever.
As will Federer's. He was simply the greatest player many people had ever seen in their lives. His talent was breath-taking and Jordanesque.
Djokovic is simply a very, very, very good tennis player. In fact, the best player in the world for the last two years. But that in and of itself doesn't make one a transcendent icon.