At the title screen press L, L, R, L, R, Select. If you did this right, you'll hear a beep.
Play as Wario and Rush
Beat the game on singles 5 times with Mario, 5 times with Luigi, and 5 times with Donkey Kong. You will now be able to play as Wario and Rush (Mega Man's dog.)
Super Serve
When its your turn to serve, throw it up with the A button, wait two seconds, then slam it with B. If you did it right you should hear like a whistling sound as it gows over the net.
Super Serve 2
When you're serving, through the ball up with A or B. Wait a second, then smash the ball by pressing Up and A as you would for an overhead smash. If you did it right, you will hear a sound like an overhead smash.
Unstoppable
If you play one of the smallest character's (Koopa troopa or Toad), you get a diving skill that you can you's by holding the direction (Left or right) and the by pressing A button you'll dive in that direction saving the ball from getting past you. Weather you be a singles or a doubles match if you stay right above the divider line you can dive and hit or tap over, even long bomb the ball as far as you can while keeping it in bounds. The only way the ball can get past you is when the computer does a everynow and then long hit from the net, which rarely happens. (even no if it get's behind you your teamate will knab it for you) Doing this will make you a unstopabble tennis pro.
Harder Game ...