Tennis can't have a rating system..
In tennis tournaments, you usually play very weak people and as rounds go by you play stronger and stronger people, the best people in the world (Like Djokovic) almost always makes finals and for sure semi's, so he would have a win/loss ratio of like 6 wins per loss. See where ELO system would not work good?
Plus they have a different points system, where each tournament gives out a specific amount of points for becoming first, making finals, semi's, quarters, etc, and then the next year you have to defend those points by playing in the tournament and bettering, or equalling your result from the previous year so you do not lose points.
It could be the same grading system as FIDE ?