Different outfits:
Hold L1 or R1 while selecting a character.

Play as King or Queen:
Win all matches without using any continues. If using a male character, you will face King. Otherwise, you will face Queen. Beat them to unlock them as playable characters.

Special rackets:
In the World Tour mode, beat opponents of level 3 or higher without losing a set. If you win a straight set you will get a special racket.

Thanks to Revolution readers Marcus and Badman!