Instructions
Hold down the "A" button on the Wii remote.
Flick your wrist or tilt your arm upward (whichever motion you use while playing Wii Tennis) to toss the ball into the air. Watch the ball carefully, and wait for the precise moment it reaches the highest point in the air.
Flick your wrist or tilt your arm downward when the ball is at its highest point to perform the fast serve.
Release the "A" button, and continue play.