On the normal difficulty setting successfully complete the arcade mode with no continues and without losing a round. If you do then you can play against Master in singles and Master and King in doubles.
Beat the Doubles Arcade Mode without continues, then beat King/Master in extra Doubles match
Play as King
Complete all 22 trial/24 training levels in World Circuit to be challenged by King; beat him to unlock
Sunflower Costume (22)
Complete Big Ball training with 10 seconds left
Fire Suit Costume (28)
Complete Big Ball training with 10 seconds left
Tambourine Costume (23)
Complete Bulls-Eye training with 7,000 points
Camoflage Costume (24)
Complete Cannon Training level with 15 seconds left
Giraffe Costume (26)
Complete Drum Shooter training with 10 seconds left
Chinese Costume (21)
Complete Return Ace training on Level 3 with 2 balls
Zebra Costume (25)
Complete Smash Box training with 10 seconds left
6/4/2001
Serve the Ball Underhanded
In a normal game, when you are at the bottom half, simultaneously press: Down, A and X. When serving from the top half, press: Up, A and X. When you press the three buttons at the exact same time it will work.
6/30/2001
Underhand Serve
All you have to do is press DOWN + A + X when serving from the near court and UP + A + X when serving from the far court.
There three buttons must be pressed at the same time. I also prefer using the D-PAD then the analog stick.