Mario Tennis Codes -

Left-handed character:
Hold L while selecting a character to make him/her left-handed.

Handicap:
Note: You may have to complete a tournament to unlock this feature. Hold R while selecting a character to make him have handicaps and make him more powerful.

Rocket shot:
Press A + B for a rocket shot. This also works when serving.

Shy Guy:
Win the Star Cup/Singles Tournament.

Donkey Kong Jr.:
Win the Star Cup/Doubles Tournament.

Donkey Kong court:
Win the Mushroom Cup/Singles Tournament as Donkey Kong.

Yoshi and Baby Mario court:
Win the Mushroom Cup/Singles Tournament as Yoshi.

Mario and Luigi court:
Win Mushroom Cup/Singles Tournament as Mario.

Mario Brothers court 2:
Win all the Doubles Tournament Cups as Mario. Note: This is another Mario Brothers Court, but not the alternate way to get the previous one.

Mario/Luigi Castle court:
Win the Star Cup Doubles and Singles Tournament as Mario.

Wario Brothers court:
Go through all the cups for Double Tournament with Wario.

Piranha Plant court:
Note: This court only works in Piranha Plant Challenge. Complete any court (perfect, all 50 balls) in the Piranha Plant Challenge with any character.

Special Cups:
Go through all the Tournament Cups with all the characters (including Shy Guy and Donkey Kong Jr.) to unlock the Special Cups, which are much harder than the previous one. Press R while selecting a character to access the Special Tournament in the menu.