Fred McNair and his sidekick, Sherwood Stewart, form one of the world's best doubles teams, currently ranked first in U.S. tennis. But their success stems from something other than their prowess as one-on-one players - McNair ranks 77th and Stewart is 73d in world singles ratings.

The pair won last year's Commercial Union Masters tournament in Houston by defeating two of the world's top singles stars, Raul Ramirez and Brian Gottfried.

That win confirmed the McNair-Stewart mastery of doubles strategy. Like two medications that taken together increase each other's potency, McNair and Stewart are, in McNair's words, "a synergetic-type team in that the sum of our parts is greater than what it should be."

How did it come to be?

"We got together by accident three years ago, in a warm-up tournament to Wimbledon," McNair said."Sherwood had been the regular partner of Dick Dell for three years. Dick took the week off and Sherwood and I teamed up. We won the tournament by defeating (Arthur) Ashe and (Roscoe) Tanner in the final. In our first year together, we qualified for the Commerical Union Masters finals in Stockholm and tied for first."

McNair's advice to would-be doubles greats: Don't search for a partner whose strengths are the same as yours.

"Look for a person whose forte jappens to be your weakness. I think opposites attract," he said. Another crucial establish effective communication. "You have to know what draws constructive energy out of the other person."

Through trial and error, determine what works in certain situations - whether your partner benefits most from a nonverbal pat on the back or from constant encouragment to keep his energy focusing on a particular strategy.

McNair said that facial expressions to ypur partner that antagonize or intimidate him."

The extroverted, free-spirited McNair and the reserved, solid Stewart complement each other beautifully.McNair is an expert at the net, his partner feels most comfortable in the backcourt.

Knowing their strong points and vulnerable spots, the McNair team draws up a game plan before each match.

"It's very important to know what you are going to do in a stress situation without having to stop, pause and think about it," said McNair, the touring pro out of the Regency Racquet Club in McLean. He stressed that being tentative with shots has no place in doubles, a game that relies on spontaneity and quick reactions.

Once the strategy is set and the game is ready to begin, many players feel that the first person to serve for a team should be the one with the most powerful serve. McNair disagrees. He prefers the stronger netman at the first.

In doubles, a game determined by which partners control their territory the best, netmen tend to stand too close to the doubles alley, leaving the server too much court to cover on the return. Servers also frequently stand either too far over at the extremes of the court or too near to the middle, according to McNair. The server should be "equidistant from the center-service-line stripe on the backcourt and from the singles line."

The most successful doubles combos over the last 20 years combined aggressiveness and power with consistency. Percentages of proper executive are enhanced by thinking first serve, first volley.

"The odds that your opponent will return the ball with a lucky of aggressive shot are reduced it you can get a controlled, three-quarter-speed serve into the backhand court and then rush the net and hit a consistent volley, instead of trying to put the ball away," McNair said.

Sticking to set plans too much, however, makes your moves predictable. The anitdote is the poach - darting along the net ot block off an oncoming return.

"It is an intimidating and agressive action," McNair said. "The safest play is to hit the poach right at the netman. he frequently overhits the ball."

On balls that either he or Stewart can reach, McNair calls "yours" or "mine," like a baseball players fielding a pop fly, to eliminate any confusion

Whenever he is in trouble or out of position, he hits dwon the middle because it is "the low part of the net, the large part of the court and you increase your chances of execution.