Here are some cheats and tips for the game Mario’s Tennis.
Expert mode:
Press Left Trigger(2), Right Trigger, Left Trigger, Right Trigger,
Select at the title screen. A sound will confirm correct code entry.
More powerful serve:
Press A + B to start the serve.
Bonus characters:
Successfully complete the game under singles mode using Mario five
times, Donkey Kong five times, and Koopa five times. Wario and Rush
(Mega Man’s companion) will become selectable characters.