Just before your opponent hits the ball when serving, press the "drop" button, and your serve return will touch the net and drop down on your opponents side.
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.
Playing Secret Characters
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.
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.
When you or your opponent(s) score a point, watch the replay. Your current score will be displayed on the scoreboard in the background, when you are supposed to be watching the play before the point was actually made. For example, your score could be 30 - 40 before the play, and 40 - 40 when you make the point. In the replay, the scoreboard will read 40 - 40.
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.
Press Left Analog-stick Up, Left Analog-stick Down, Left Analog-stick Left, Left Analog-stick Right, Black, or White. For more taunts, press Black(2) for frustration or White(2) for excitement. Press White(3) after missing a point to throw or kick your racquet.
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.
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.
If your opponent attempts to serve with a risk shot, watch his or her meter. If the bar stops above the blue section of the bar, it will be out. If it stops below it, it will hit the net. This is an easy way to determine whether you should just stand your ground or get ready to hit the ball when he or she serves it. As long as you tap the button as soon as your opponent hits the ball, you will always hit it back. Sometimes the risk shot even stuns them for a moment, keeping them from hitting your ball back in time as long as you aim the ball away from them, and you can make an easy point.