To unlock Duke, win the Singles Matches in Tournament mode without losing once to the CPU and get a flawless "C" or better rank. Defeat the Duke when he challenges you to unlock him.
Unlockable Rackets: Unlock the following rackets by performing the corresponding tasks:
RACKET - HOW TO UNLOCK Frying Pan racket - Get three strikes in a row in the Pin Crusher mini-game. Guitar racket - Defeat King or Duke in a SPT final. Tambourine racket - Make 100 lob shots. Wooden racket - Win King Of Players mode.