If you are talking about male tennis players specificaly ,then Martina Navratilova is the greatest ever.
But in all honesty, this must be the worst sports poll I have ever seen ! even non-tennis fans know of players like Borg, laver, Lendel and Jimmy connors that should have been on your poll first !
The soccer equivalent of your poll would be: what is the greatest soccer country ever:
Brazil,
Holland,
Greece,
none-J.J Okocha