Every now and then press the left trigger to do a drop and hit serve and catch your opponents off guard.
When serving, don't go for the super powerful serve with a low percentage of getting it in because the AI will usually get it back anyway, go for the consistant Slice or Topspin serve outwide or down the line and you'll win many more matches.
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. Additionally, the best way to win quickly is to do a very weak serve to the outside. That will get the CPU player off to the side. You can then come into the net a little and smash the ball to the other side, winning the point.
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 use Y (Lob) to win.
When your about to serve pull the left trigger as a drop shot serve and when the computer hits it push the Y button to hit it over his head! He rarely gets it!
IF you want to get your Serve at it's best, here's what you do: Make sure you get as many star's as you can for it, even all gold, because in Tornaments you need the serve more than anything!
To play as some secret characters, use the random feature of the game; the game will choose one of the career default players instead. Keep pressing B then A until you get the desired player. By doing this you can play as players such as Raquel Mello, Candy Spoon, Karen Archer, and many others.
Press the Left Thumbstick Up, Down, Left, or Right and press Black or White
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.