I played tennis collegiately and it doesn't really matter. It's really easy cut off angles of attack and you don't really have to cover as much court as you think as long as you understand geometry.
Technique, mental fortitude, stamina, and such are all way more important than athleticism when you get into top level tennis players. Boxers tend to pick up tennis very quickly.
Actually, LeBron's footwork is pretty solid here. You like the angle at which he's facing the court relative to the ball; open stance; front foot extended. Nice, open racquet head and arm extension to deliver accuracy and velocity. Ideally, though, he would bend his knees to lower that strike zone 5-7" so as to generate more power out of his core.
