FYI, unless you don't like your knees, don't do leg extensions. Ask any good personal trainer, they wreck knees at even low weights. Do squats or leg press instead, and don't neglect the hamstrings ... for optimal knee health and performance, you want your hamstrings to be matched in strength to your quads ... not easy to do but makes a world of difference.
1. Play a lot of tennis
2. Plyometrics ... with and without bands, including lunges with and without weights
3. Play more tennis
Get enough protein in your diet to be able to build muscle.
Also, not everyone will genetically be able to build large muscles .... wouldn't it be better to just be the strongest you can be without worrying about size?
BTW, I am one of those that builds muscle stupid fast, it makes shopping for clothing a real pain.