The surest sign of a Washington Memorial Day used to be the shriveling of the azaleas and the disappearance of everybody to Ocean City. The sign is different lately, but just as sure: Those bodies surging onto local tennis courts.

Never have so many hacked so often with such glee. Never have the great and near-great worried so fervently about calcium deposits on their elbows. Now that pictures of tennis players adorn the walls of Duke Zeibert's Restaurant, amid various almighty Redskins, little more proof seems necessary.

Not that every player here is ready for Wimbledon - some local weekenders look ready for the morgue instead. But the waiting time for courts and the vast sums being spend for equipment argue that something is going on out there besides gardening.

And it is something very much better, in terms of tennis skills, than it is used to be.

That from a native son who has gone to be a prominent professional. He is Fred McNair IV, of Chevy Chase, half of a doubles team that just missed winning the Italian Open title last week and is currently ranked second in the world. When McNair says Washington tennis has improved "more than just about anywhere else," he is speaking of where he learned the game.

McNair learned it specifically at suburban Washington country clubs, and he cites the spread of the game to public parks as the major reason the area as a whole has bounced upward in skill.

"I was practically raised at Columbia Country Club," said McNair, 26. "But there and the Edgemoor Club were the only places to learn the game. They were the only places developing good junior players."

Now, two public high schools, Walt Whitman of Bethesda and Langley of McLean, have varsities ranked in the top 10 nationally. And kids who might have been attracted to football, basketball, baseball or cheerleading in another era are devoting themselves exclusively to tennis as early as the age of 6.

Meanwhile, every public park has its reigning terror. As Fred McNair notes, even definitions have changed. If you happen to answer an ad on a bulletin board for an "intermediate" doubles partner, he may turn out to have been a former star - and he is likely to be very unforgiving about your backhand.

"Tennis," said McNair, "used to be a game just for the Mercedes of Washington. Now it's a game for the Vegas, too.

"I think it all started to change here in 1968, the year tennis went 'open.' There was no longer any so-called 'shamateurism.' That was when professionals got money under the table. Now it's much like any professional sport. Grass-roots kids saw an opening."

But adults have produced the Washington tennis explosion, and McNair thinks the reason is health-consciousness.

"There's such an emphasis on it nowadays. And there's nothing like tennis to accomplish those ends. In one hour's time, you can get a lot done.

"Plus it's social. You may not be able to hit the ball worth a damn, but it's fun."

It has also become a big moneymaker.

Nine years ago, "name" pros only came to Washington when they were changing planes. Now, both the men's and women's professional tours stop here each winter. In the summer, the Washington Star tournament, played at a public park in Northwest Washington, draws many of the "name" players in the world.

What's more, the three tournaments often sell out. And the people wouldn't come out to see it if they weren't playing the game themselves, McNair noted.

Where they are playing is at the area's indoor clubs, most of which have been built in the last seven years.

Playing time at most is not as expensive as the wall-to-wall carpeting in their cocktail lounges might suggest. Prime time (evenings) can be as low as $6 an hour. Split four ways, that's cigarette money. Many of the area's 18 major indoor clubs are even busy in the summer, thanks to their August-defeating air conditioners.

Along with the clubs have come the pros.

The top seven young ones in Washington all have teaching or "residence" affiliations with a club, and all were once ranked nationally in either singles or doubles.

It is a chicken-and-egg question, of course, as to whether the pros followed the interest, or vice versa. McNair suspects the interest came first, and he points for a reason at Washington's competitive character.

"Tennis tends to be an outlet for hostility sometimes," McNair said, and if hostility wasn't first generated in Washington's power corridors, it has been perfected there.

McNair is less certain about how solid a paying proposition Washington tennis will remain.

"I do think there's a limit," he said. "Right now, Washington has not reached the saturation point. But it could, and it could soon.

"The danger is more to the investor than the public," he said. "The investor is going to have to accept the fact that courts lie fallow.

"It used to take only five years to recoup your original investment on a major facility. Now, the return is going to be spread over much more time."

One major facility in McLean, for example, opened five years ago. It offered the works: training rooms, ball machines, even videotape cameras so one could watch one's own hopelessness. The place is still open, but according to McNair, who once owned a "piece," it has never made money.

"The lesson may be that some people just want to get out there and hit, not study the game to death," he said.

But in Washington, the hitting is so much more expert than it used to be that just being good might get discouraging one of these days.

"My brother John (15) is a very good player," said McNair, by way of example. "He's probably better than I was at his age. But he's ranked only fifth in the Washington area.

"That says a lot."