On his TV show last winter, Sonny Jurgensen asked Arthur Ashe the most important rule of playing good tennis. "Get your racket back," replied Ashe without hesitation.

"Okay," responded Sonny with a telling, guilty grin, "what's the second most important?"

"Follow through," said Ashe, his placid pan still dead.

Nothing is more important in the finely tuned, fast-paced business of hitting tennis balls then staying close to fundamentals. The great danger in watching pros on TV is that they make it all look so easy and often seem to violate the rules you've learned from your early training. The truth, as Arthur Ashe demonstrated, is that the greatest players never stray far from fundamentals, even if their strokes all look very different.

One thing all those different groundstrokes have in common is that they grab the ball for a split-second at contact. The ball is not slapped back; it is very briefly held on the racket face like a lacrosse throw, thus imparting greater spin and control to the ball. This is why your wrist should be "locked" like a block of granite at the moment of contact.

One way to keep the ball on the racket face longer is to imagine you are hitting more than one ball (one right behind the other) every time you swing. The average player hits two or three balls with each stroke; Jimmy Connors hits a whole tunnel of balls.

Good balance during the shot is important, too. Move continually with short, ballet-like steps into stroking position. At the moment of contact, imagine yourself in a still photo - your head and eyes centered over the ball like a golfer on his tee shot. Hit a whole tunnel of balls.

Develop a fluidity around the court. Your backswing should begin the moment the ball leaves your opponent's racket face. Take the racket back in a single smooth movement, not in separate jerks. Your follow-through should be up and over the net.

A common mid-summer complaint involves the breakdown of a person's backhand. This is often caused by what we call "elbow backhand" - the elbow is leading the wrist and racket into the stroke, which usually results in a mis-hit or weak shot. It's the scourge of beginners and teachers alike. Make sure you're keeping your elbow tucked in close to your body and follow through with both arms and the racket up in the air like a bird spreading its wings.

Q - My forehand errors are usually long. Why is this?

A - Check to make sure you don't pull your head up too soon during the shot. You may also be swinging the racket like a golf club, starting too high and then swooping too far under the ball.

Q - I'm afraid to follow through completely because I don't want to hit the ball out. What should I do?

A - The follow-through controls how deep you hit the ball, but it won't go out if you aren't committing any other faults. Start following through naturally - it's healthier to hit long than into the net.

Q - I have trouble with groundstrokes off high bounces. What can I do?

A - Bring your racket and shoulder up higher on the backswing and punch the ball back. Don't try to hig down on the ball or go for spectacular winners. CAPTION: Picture, GOOD STROKES START THE INSTANT THE BALL LEAVES YOUR OPPONENT'S RACKET. By Charles DelVecchio.