If I'm not mistaken, they are the overall fittest athletes because of what they do.

I think so. They have to do so many different kinds of movements and have to be flexible through every joint too.

yes, extreme - but when they are training they are training. Djokovic doesn't hit around with some kid on the weekend and consider it quality training, but it might be fun. They all kick back at times, but their training is serious business, for most of them at least.