The three most important things in tennis are:
1) Watch the ball
2) Watch the ball
3) Remember the first two things!!
All life revolves around the ball when playing tennis. Nothing even comes close to being as important as looking at the ball when you play. Your eyes should be on the ball at all times – not on your racket, not on your feet, not where you are aiming, not at your friends watching, not at your opponent, and not at that bird in the sky
First of all, looking at the ball when you swing at it will, obviously, allow you to hit the ball in the center of the strings of your racket. Be sure to remember to keep your head still when hitting the ball – if you move your head, your eyes move; if your eyes move, you can’t see the ball; if you can’t see it, you can’t hit it!! Look at any picture of Roger Federer hitting and you will see his head and eyes glued to his contact point long after the ball has left his racket.
Secondly, always be watching the ball come off your opponent’s racket. You will be able to tell, instantly, how the ball will be coming to you – is it coming to your forehand or backhand; is it coming high or low; hard or fast; short or deep; spinny or flat; what kind of spin; or a combination of all of these.
Thirdly, watching the ball as it is coming to you – really studying it – will allow you judge what distance you should be from the ball and will let you adjust your footwork to position yourself correctly when you swing at it. When to stop running and set up to swing; when to hit the ball while running; how far to back away from a ball hit right at you – these are all functions of watching the ball.