If a football player can enter the game with a broken arm in a mummy case, and basket-ballers play with advanced pneumonia, why do tennis players sometimes default over a blistered thumb, as Bjorn Borg has done? Are we really just sissies in short pants, as somebody used to say?

Not quite. Harry Hopman, the great Australian Davis Cup coach, used to impose a rule on his players - Rod Laver, Roy Emerson, John Newcombe and other notables - that "If you're injured, don't play, and if you play, you're not injured." That's a fine fiat for the hardened professional from Down Under, but what about your basic neighborhood hacker?

Tennis injuries that seem slight can be serious. A blister can become torn skin and a running sore if not properly treated or kept off the tennis court; a wrenched back can do a lot worse. Another reason tennis players can't make like a rugby scrim and play through every form of pain, bloodiness and loss of teeth is that tennis is a game of fine concentration. While you can never feel physically perfect, you have to feel good enough all over to play your best. A toe blister that slows your rush to the net can be enough to throw your entire game off. Without concentration, a tennis player might as well be playing tiddlywinks.

The most common complaint is tennis elbow, whose most common cause seems to be uneven backhand strokes - usually meeting the ball late. At the first sign of elbow pain, try taking a few days off. You might also consider your equipment: If your racket is srung extremely taut (over 55 lbs.) of if you are using heavy balls (Tretorn or Slazenger), this could aggraviate your problem.

One of the most intracable tennis injuries is back strain and pulls. Apart from seeing your physician, you should try a layoff until any back spasms have subsided. Do exercises to strengthen abdominal muscles, such as sit-ups, thus taking the strain off the back; try to duo hip and back exercise on a Nautilus machine). You should, of course, make it a rule to stretch and flex your back before and after you play. Swimming is usually recommended as an all-around antidote to back problems.

On the court, the back sufferer should forget about using any twist serves, which is a real sacroiliac-wrencher. Go for slice instead: Toss the ball out farther in front of you and swing across it without arching your back at all. Adopt a more canny, less straining style of play: lobs, moonballs, dinks, sharp angles and dropshots. In short, by keeping an open mind and showing some consideration for your body, you can learn to play around your injuries.

Q - I have a bad case of tennis elbow, but I can't resist the urge to play. Any suggestions?

A - You might try wearing an elbow support. Elbo-Aid makes one that comes in adjustable sizes. To take the strain off your backhand, try hitting the ball with both hands, as Jimmy Connors does. Whenever possible, run around your backhand.

Q - I am prone to muscle pulls and strains in the legs. How can I avoid that problem this year?

A - The single most important thing, especially if you are over 30, is flexibility. Do stretching exercises. The easiest and most common is to lean against a wall, tree or fence with your feet flat on the ground for two minutes every day. This is especially important right after you finish play. Arthur Ashe, who at 35 has just proved that old tennis players need not fade away, now does a full ten minutes of stretching after every match.

Q - I recently hurt my back in the middle of a match. I decided against defaulting but wonder if I aggravated the condition?

A - If your back still hurts a lot, you probably did. Harry Hopman's boys would not have defaulted in this situation, but the medically sound thing would have been to get of the court as soon as possible.

Q - I have frequent problems with blisters. Is this normal?

A - When starting out the season, blisters are normal. With frequent play, they should turn to calluses. Be sure you are not gripping the racket too loosely. But at the first sign of a red spot, apply adhesive tape, which can be carried in your tennis bag. Rod Laver sometimes played great matches with tape on five fingers.

Q - Is there any way to keep an injury-prone back warm throughout a match?

A - Pros Dick Stockton and Tom Gorman use a rubberized corset, available at diving shops, to keep their backs warm and sweaty when they play. This reduces the possibility of a muscle pull. CAPTION: Picture, no caption