Answer:
Newton's law of inertia
Step-by-step explanation:
This states that, a body will continue to be at rest or moving at a constant speed in a straight line, unless it is acted upon by a force.
The force could be internal or external.
Since the player's one hand can't produce the desired effect on the tennis ball and his two hands can produce more force to put the tennis ball in motion, then the law of inertia is being obeyed here.