Select 'Single' match type in Arcade mode and win ALL four matches without losing any games. You must also not go to 'Deuce' in any of the games. When you accomplish this you will play a special match against King. Defeat King to unlock him as a playable character.
As a male tennis player complete Arcade mode without losing a game. When you accomplish this an exhibition match will appear at the end of Arcade mode with a special match message stating your next opponent will be Duke. Defeat Duke to unlock him as a selectable character.