Both Michael and Trevor can play tennis and there are eight courts to play on around Los Santos. Having a higher strength stat will allow you to hit the ball harder. This will help a lot, especially when playing harder opponents.
Start off against an easy opponent so you can become familiar with the controls and feel of the game. The key to success is to use enough variety on your shots to keep your opponent on the back foot. Use different shots, different spin and different directions to really stretch them.
When serving, try and get the bar to stop right at the top so you get the most power from your serve. If you aim it fairly wide, you can get an ace but even if the opponent gets it back, it will normally come back short, allowing you to move up and smack it away into a corner. This works as a great one-two punch and also keeps points and games short.
When receiving you can hit drop shots to force your opponent into the net and then you can finish them off by executing a really hard passing shot. Using topspin can also cause the opponent to either hit it out or a little short. Keep the opponent guessing.