Originally Posted by WildVolley
You can make your muscles leaner by losing strength, but I'm not sure how easy it is to make your muscles longer by lifting lighter weights.
My personal opinion is that tennis players should focus on training like a sprinter in the weight room, but with more focus on rotational exercises and shoulder/wrist work. The weight room isn't the place to seek endurance. Tennis relies on fast twitch muscle fibers for quick response and racket head speed.
If you want to change your body composition, diet is going to be the most important factor. The extent to which you build big muscles or have a lean look will mostly be genetic.
IMHO - It's pretty easy to train for endurance using light eights and high repetitions. Exercises like lunges using body weight, medicine balls, or even a light weight or empty bar are a good strength training exercise for tennis that can contribute to endurance. You don't have to do that in the gym, but you can.