Sega Sports Tennis Unlockable: Tank Racket
Complete Tank training at least till level 4 and you will get the special Tank Racket.
Sega Sports Tennis Unlockable: Alien Force Racket
Beat the level 4 Alien Force Volley training.
Sega Sports Tennis Unlock King or Queen
To unlock the King or Queen, win all of your matches in Arcade mode without using continues. If you’re male, you’ll face King. If you’re female, you’ll face Queen. Beat them to unlock.
Sega Sports Tennis Unlock King’s Court
To unlock King’s Court, defeat the King and Queen in Mixed Doubles, in World Tour Mode (level 5).