Another simple trick to defeat difficult opponents is to use drop shots regularly. Do a drop shot when the opponent is a bit far. He or she either will not get it or reply it; then you can just lob to win.
When you are serving, look at your ITZ meter. If the ITZ text is flashing, this indicates a 100% chance that you will serve the ball with 100% accuracy using the risk shot. You can easily hit the ball over the net at speeds of over 108 mph. You only need to tap the button. It will automatically give you a solid risk shot.
Recommended tennis racket:
It appears that the tennis racket you can purchase for 5,000 coins has different effects depending on how much power and accuracy you have on your swing. Although there are no stats regarding the tennis rackets you can purchase (abilities, strengths/weaknesses) the tennis racket that you start with is by far the best racket you can use.
The simplest trick to defeating every possible opponent is to first hit the ball to one side. They will run and hit it. Then, hit it back hard in the other direction. They will probably make it, but dive or run too far. If they do, smash it to the other direction. They will never hit the ball.
This trick works on every opponent.