Simply put Lefty, training is goos anytime, it just depends on your approach and diligence.

For fat loss, a workout before breakfast works a treat.

As regards meals, I presume a high protein, medium fat, low carbs diet would serve you well.

In the beginning aim to drink as much watere as you possibly can, your body will monitor the level of fluid in the body itself and better too much than too little.

Using a stability ball, you could work on core strength, which would increase balance and overall body strength, ideal for tennis right?

At the end of the day Lefty your aim is to specialise your body for tennis, so everything you do should just be supplementary to your tennis.