The greatest ever tennis player is Novak Djokovic. There is zero doubt about that. That is about success.
The greatest ever tennis champion is Roger Federer. There is also zero doubt about that (in my opinion). That is about comportment, which is very important in the sport of tennis.
ps Success also comes under thinking about who is the greatest ever tennis champion. I cannot skip Roger Federer here and mention somebody who hasn't won a whole lot but who has nevertheless had, say, impeccable comportment. And that is very, very importantly because comportment has to do with challenges faced.
pps Roger Federer's comportment has not been perfect. But he is the greatest ever tennis champion (in my opinion). And Novak Djokovic is a great champion too. It is just that he falls very, very, veery much short of Roger Federer in that specific regard.