BECAUSE professional tennis is played worldwide and even the 100th ranked player commands a bit of fame, almost all of the pros earn extra money through endorsements. These endorsements can start with rackets only and run all the way to the megabucks some superstars earn from using certain equipment and/or licensing their names.

Tennis players are unique in this regard. No other group of athletes in any sport has as much marketability so far down the line.

The people who put together these contractual arrangements are agents. Trouble is, anyone can be an agent. There is no state or federal licensing of agents.

Furthermore, there is no such thing as a legally binding confidential agent-client relationship. What you tell your doctor or lawyer is your business.But what you tell your agent is not privileged information.

The fact that anybody can be an agent without passing a licensing exam and the nonconfidentiality of the agent-client relationship have combined to produce more problems for athletes than they have solved.

Tennis players are marketable because they wear clearly distinguishable clothes or shoes, use identifiable rackets, or teach at certain clubs. but if you play a team sport such as basketball, baseball, soccer, football or hockey, equipment endorsements are harder to come by-unless you are an acknowledged superstar. So the agents these team athletes utilize are valued more for the salary deals they can make with team owners than for their skills in securing endorsements.

Having said all this, we now bring in the wide-eyed, innocent, fourth year All-America draft choice from an NCAA Division I university.

It is likely he did not graduate. If his sport is baseball or hockey he may not have attended college at all. And basketball has a disproportionate number of hardship cases. They are the highly talented players who come from poor families and want to turn pro before finishing college.

In order of vulnerability and naivete we go from the 18-year-old hockey or baseball whiz kid who'll never go to college (most vulnerable) through the hardship cases to the first round draft choice who was graduated from a major university (least vulnerable).

Enter Mr. X, agent, with a proposition for the future pro athlete. The student-athlete is immediately at a disadvantage for two reasons. He has no clue as to what his value is to a team owner; and being 22 years old or less, he also has no idea what he will be doing when he's 35.

What is needed at the outset of their relationship is some counterpart to the Miranda Decision. Just as police read a suspect his rights upon arrest, so an agent should inform a potential client what he, the agent, can and cannot do for him.

Money talks. Aware of this, Mr. X knows that a guarantee "up front" can do wonders. This sum is usually high enough to impress the prospect, but not so high that he can't deliver.

By now the young hotshot is ripe for a little double-dealing. If Mr. X has had previous dealings with Mr. P, team owner, and gotten to know him, Mr P could make a side deal with Mr. X to go easy with the salary demands.

Even though Mr. All America may pay Mr. X to represent him, Mr. X may wind up working for Mr. P too and being compensated from both at the same time.

I am not suggesting that this happens every day, but it could.

On the other hand, reputable sports agents and sports lawyers in particular, get a bad reputation because of a few dishonest agents. The public seemingly has lumped the good and bad together.

The athlete has to be able to separate the good from the bad.

Sports lawyers have been especially sullied because of an unwarranted connection with some dishonest agents. Lawyers hate being called agents. Even though the word agent is generic and could apply to some lawyers, the lawyer thinks of himself as several cuts above agent status.It is also odd that the athlete's lawyer is always called an agent while the team's lawyer is a lawyer.

Highly successful sports law firms wind up more powerful than any of their clients. Sports Illustrated described Mark McCormack two years ago as the "most powerful figure in sports." His clients include Arnold Palmer, Gary Player, Bjorn Borg, Vitas Gerulaitis and Jean-Claude Killy.

He also has a subsidiary that produces sports packages for network television. This gives him tremendous leverage both with his clients and with the network because television cannot package some sports events without his clients, and his influence with the tube helps nail down potential clients.

The college athlete is really taking a chance if he turns away from such a firm and chooses instead a family friend, a local banker, or his parent to be his agent.

Hometown people may be very honest and aboveboard, but modern sports representation is very specialized.

Firms may charge more, but a higher percentage of something substantial is better than a lower percentage of something smaller.

Here is some advice to potential pro athletes seeking representation:

Don't negotiate your own contract-you'll be doing yourself an injustice.

Talk to at least three respectable firms.

Be wary of people who are not lawyers-though there are certainly some very good agents who are not lawyers.

Include your parents and/or guardians in these discussions.

Think Long Term-where do you want to be when you're 35? If you're not sure, take an educated guess.

Ask the potential agent to explain to your satisfaction the option clause, the reserve clause and free-agent status as it relates to your sport.

If you select a firm and compensate it on a percentage basis, get an itemized list of services to be rendered. If compensation is on a cost-plus free basis, agree beforehand what items are "costable."

Sign all your own checks over $500.

If you give your agent your power of attorney, make it specific rather than general.

Read every contract before you sign it but once you sign your name-honor your commitment, come what may. CAPTION: Illustration, no caption