THE TOUGH THING about friendly games of tennis is that some people play poorly and some play better, and when they try to play each other they get mad an sad.

The tennis boom has made things even worse, because now 10 duffers, if that's the word, are to be seen misplaying shots where there was one one before.

When the poor player faces the better one, the poor player loses, which is not fun. And the better player wins more or less by default, which is not fun either.

When the boomer tries to carry the blooper, out of pity or just to prolong the exercise, the result is even more disastrous. Budding friendships are nipped.

It has something to do with the divorce rate, and the divorce rates has something to do with it. Anyway, there are a whole lot of singles and couples out there who would like nothing better than a friendly game of tennis, if they only could find friendly persons of similar ability.

But how to begin? When strangers agree to play, how can they arrive at fair pairings? In 25 words or less, how good, or bad, is your game?

Comes now Otto Bernath with what he says is the answer.

"You need a handicap, as in golf.

You put a number on the player's ability that means the same thing to all players everywhere. Then when players of unequal ability face each other, the better one spots his opponent a certain number of points and they start even."

Bernath, 56, a Sperry-Univac executive, has developed such a system and predicts it will sweep the tennis world.

It is called The United States Handicap Tennis System. The acronym (TUSHTS) is rather unfortunate, but the system has proved itself in trials with more than 2,000 players from world-class pros to raw beginners, he said.

"The beauty of it is that it's so simple. You can handicap 100 players or more for a tournament in a matter of hours," Bernath said."And when play is over they will all have had fun."

Although Bernath was a math major and is a computer person, the system is a matter of simple arithmetic. He demostrated it with a pickup group of six players at the Regency Racquet Club in Mclean.

"It almost works itself," he said as he handed out guide sheets telling each player how to rank his ability on a scale of 0 to 5 in the areas of serve, ball control, and repertoire of shots. The numbers are added to give a rating of 15 for a ranked player down to 0 for somebody who just bought his first racket.

The ratings of the six players Bernath had roped in came out 7,7,6,6,4 and 3. Bernath looked suspiciously at the self-proclaimed 4, who confessed to not having played in two years and never having had a lesson but who was immoderately proud of his serve.

Bernath then proceeded to mix and match the players in doubles.

When a 7 plays a 6, he spots the 6 one point per game, and we use five point games. A 6 would spot a 3 three points, so that if the 6 loses two points before he wins five, he's lost the game.

"In doubles you total the handicaps of each pair, subtracting the lower. If you get fractions they are ignored. The maximum point spot is three; players more mismatched than that really shouldn't be paired, although we've done it many times with pretty even results."

The play was short and brisk.

"That's another great thing about this system," Bernath said. "You can play most matches within an hour, and with court time as expensive as it is, you get back the handicap fee ($1.50 for individual players, $1 for club members) in about 10 minutes.

At the end of the first round of doubies, among the two 6s and the 4 and 3, the 6s were 7-5 and 5-7.The 3 was 9-3, and here husband, the 4, was 3-9.

I thought you were overrating yourself," Bernath told the 4. "And your wife is too modest; she has a very consistent game. I'm going to reverse your ratings."

Then, playing with the 7s, the promoted 4 wound up 4-8 and the demoted 3 went 6-6. One of the 7s was also 6-6, and the other was 8-4; several of the games went to tie-breakers at 4-all, in which the receiving team designates the service court and the next point decides the game.

All the players were enthusiastic after the matches.

"This is the first time I've played against an advanced opponent without feeling humiliates," the 3 said.

"And I lost a couple of games to him without giving them away," one of the 7s said.

TUSHTS rankings have been adapted as the format for the National Indoor Tennis Association's annual tournament, and are being used in a current 10-club tournament in the Washington area.

"The more people involved, the better it works," Bernath said. "We had a tournament in Arlington in which all top four places were decided by tiebreakers.

"And it works just as well for individuals if you make adjustments as their game develops. I started a raw beginner on the system last year and at the end of the season he was 60-60 in his matches.

"The system doesn't equalize players, just the scores. If you're on top of your game one day, you win; if you're hung over the next day, you lose."

A system that works so well might be expected to have deep theory behind it and many hours of computer time in its pedigree.

"No," Bernath said. "I worked it out with a pencil in my spare time over a couple of weeks for an office tournament."