What is the best drink to drink while tennis training?

This depends upon your goals. If performance is your only goal, then start sipping on a sports drink like Gatorade / Lucozade, etc about 40 minutes into the game. Many pro tennis players eat and orange or banana but a sports drink is a better choice as it ads hydration and has a better source of carbohydrates

If recovery is important, you could add some whey protein, hydrolysed whey protein or essential amino acids to the drink and start drinking it just as you start playing.

I you are playing as part of an activity plan to help stay lean or lose fat, then have lean protein or a protein shake about an hour before and just after the match and just stick with water during activity unless you find that you run out of energy or get shaky etc.