Instructions
Hit the ball at the correct time. One of the crucial components in spiking the ball is timing. When your character throws the ball up, swing your arm that is holding the controller in a downward motion when the ball reaches its peak.
Use force and speed when moving your arm. Just as in real tennis, the faster and more forceful you are when spiking the ball, the better your serve and spike.
Press the "A" button when the ball reaches its peak. While you are moving your arm in a downward motion, pressing the "A" button simultaneously will complete the spike.