Unlock King:
     Select Tournament mode and win the singles matches without losing
to  the  CPU  and get a flawless "D" or worse rank. When challenged by
King, defeat him to unlock him as a playable character.

Unlock Duke:
     Select Tournament mode and win the singles matches without losing
to  the  CPU and get a flawless "C" or better rank. When challenged by
Duke, defeat him to unlock him as a playable character.

Unlock Frying Pan:
     To unlock the Frying Pan as a tennis racket, get three strikes in
a row in the Pin Crusher mini-game.

Quick Volley skill increase:
     In Career mode, play the "Defend The Prize" mini-game repeatedly.
Choose  one prize and stand in front of it (ignore the others). Simply
defend  that  one  prize. You will easily win the mini-game every time
and be able to increase your Volley skill as quickly as desired.