Red bull has too much sugar and that will dehydrate you, so I wouldn't recommend it. I would drink gatoraide for rehydration, plus it replenishes electrolytes that aren't available from plain water. Some people say you should drink gatoraide in 1/3 concentration, because that is the closest to physiological concentration. However, you don't absorb 100% or the electrolytes anyway and it taste way better undiluted, so I just drink it straight.
Some atheletes will take caffine prior to a excercise that requires lots of power. Or, people trying to lose weight may take it because it allows you to burn more fat(this is why it is a main ingredient in weight loss supplements like metabolife). Tennis probably fits into this power requiring kind of activity. However, caffine, one of the main ingredients of cocaine, is not healthy especially when taken often or in large doses. So, I wouldn't recommend this either.