You should be drinking water every 20 minutes in a game of tennis. Even while sitting still, your body loses water, so just imagine the amount of water being lost while running on the tennis court!? It is even more important for junior players to keep hydrated through games, as lack of fluids can cause a high amount of lapse in concentration and focus, physical fatigue and sometimes even muscular pains. 

Bonus tip: Adults should be following the same regime to ensure that their game stays to the best of their potential. Adults are also more prone to injuries so high in-take of fluid can help those tired muscles last those extra tough games and help outlast your opponent!