Learn From Balanced Characters
"Mario Power Tennis" offers more than a dozen characters and each one plays differently. Each character has obvious differences, such as their distinct power moves, but the most important differences are less obvious. Some characters such as Bowser are much slower than the competition. Others can't reach far with their racket. Those that excel in one ability often are poor in another. The most well-rounded characters include Mario, Luigi and Peach -- appropriately enough, the heroes of previous titles. Learn the basics of the game with these characters, and once you feel comfortable with them, begin trying others. Specialized characters are harder to play, but once you've mastered them, they're actually better than their well-rounded counterparts.
Learn to Use "Tricky" Characters
One of the many attributes a character has is his "Tricky" score. Characters with high "Tricky" are the most difficult to master, but they also have a unique ability: they add spin to their hits. This makes predicting and countering a shot from a "Tricky" character more difficult. The trickiest tennis players in the game include Boo, Bowser Jr. and Fly Guy. These are the penultimate characters, but only a master of "Mario Power Tennis" has a hope of using them well.
Only Use Low "Speed" Characters in Doubles
Bowser, Wario and Donkey Kong might be strong, making their shots difficult to counter, but they're also absurdly slow. "Mario Power Tennis" demands that you move quickly around the court to counter shots from your opponent. Because of this, these slower characters tend to lose in singles matches. However, when teamed with a fast character in doubles, they make a powerful addition. The fast character can catch stray balls while the powerful one devastates anything that comes into reach.
Gimmick Courts
Gimmick Courts completely change the nature of the game. Some courts swivel back and forth while other unleash ghosts and man-eating plants on you. Because each Gimmick Court is different, you should play through them several times to get a feel for how they operate. Some courts are best played with particular character types. For instance, fast characters are best in Luigi's Haunted Mansion because you'll need to not only hit stray balls but also avoid ghosts and maneuver around bananas. In contrast, Wario's Factory is best played with high-technique characters. By aiming at and successfully hitting the various buttons on the court, you'll be able to operate your opponent's conveyor belts, making it harder for him to get around the court.