I agree with the post above and also like to add a few things for "Tennis specific training". You need to train with high weight and low-mid range reps (8-15) BUT on unstable environment. Tennis is a sport that involves speed, and quickness which can only be achieved by having good balance. For example instead of doing barbell bench press (stable environment), you may want to do one-arm dumbbell bench press instead to increase core strength and stability which in turn will give you better balance=more speed and power. As you get better you can use a stability ball instead of the bench for progression.
We also need to focus more on the muscles that slow down the movements which I called "Breaking system". This will allow your body to move even faster to the best ability. Imagine a Ferrari with really bad breaks racing with a highly tuned Honda with an excellent breaking system around the track with many sharp/short turns...who do you think is going to win? For example we use our chest, shoulders, abs, hip flexors , quads and many other anterior muscle for our big forehands. The muscles that slow down this movement are Lats, rear delts, glutes, hamstrings and many more posterior muscles. If we put so much focus on how much we can bench, that will lead to muscular imbalance and injuries.
Also don't forget about flexibility...