Little Cindy Harris, a seventh-grade ballgirl, knows who the real champions of the Virginia Slims are.

She and several teenage companions were assigned the job of guarding the women tennis players' locker room door at Smith Center this week.

They know who's naughty and nice, who's grumpy and sweet, who's aloof and who's friendly to the towel-fetchers and errand-runners.

"Hey, hey, hey, here comes Jo Anne," they all whispered, as Jo Anne bounced down the subterranean steps after beating Regina Marsikova yesterday to reach the Virginia Slims of Washington quarterfinals, 6-3, 6-3.

As Russell went through the locker room door, a half dozen kids jumped out cheering and saluting and patting her on the back.

Martina ("Nobody can beat me in Washington") Navratilova, Betty Stove and Dianne Fromholtz also won yesterday to gain the quarter finals. But no one received a genuine testimonial afterward like the one Russell, got from the children she calls "my marching band."

"Hey, hey, hey, didn't letcha down, did I?" said the 23-year-old Russell, who would be any kid's dream of a big sister. "All right now, I expect another six-gun salute tomorrow, too."

"Don't forget," grinned 12-year-old Cindy, "we knew you back when."

It is still back when for Jo Anne Russell.

She has won Wimbledon - in doubles.And she was 13th in the world in money winnings last year, $55,516 - most of it also coming in doubles partnership with Helen Cawley.

But mostly Russell is unknown. That is, except below stairs, where she is the friendliest, funniest, funkiest tennis pro ever to blow a match and then gasp, holding her throat, "Gawd, didya see me choke that like a dog?"

She doesn't lose matches, she gets "killed, crushed and mutilated."

When she wins, she greets her tiny media gathering by saying, "OK, let's call this mammoth press conference to order. Who stole my gavel?"

Russell knows she will never be great. As a junior, "evfrybody beat me," she says, then lists a dozen of her anonymous conquerors. "Of course," she adds, "they're all out of tennis now and I still appear to be here."

Nevertheless, a modicum of singles fame may be on the horizon. (Stop rolling your eyes and giving that Bronx cheer, Jo Anne.) The Slims of Washington is her first tournament since new coach Dennis Ralston made major alterations in her sweat-and-struggle game.

It has been an auspicious start. Granted that the draw - Francoise Durr, Marsikova and now 35-year-old Nancy Richey in the quarters on Friday - has been merciful.

And Bille Jean King, whom Russell resembles in everything except talent, has already been knocked out, giving Russell as clear a path into a Slims final as she is ever likely to get.

Nevertheless, it is a dramatically more confident, fit and well-rounded Russell who begins 1978, winking at her fellow players and saying, "Did you catch my new game? What do you think? Is Chrissie (Evert) in trouble, or what?"

Ralston has altered "kinda basicstuff," says the Trinity (Tex.) graduate. "Like he taught me how to serve so I didn't look like the Statue of Liberty toppling over. And, oh, yeah, he taught me how to run. I must have missed school the day they taught running."

Yesterday Marsikova ran into Russell at her scramblling the best.

"Regina worried me with all that big topspin. Lord, they sure know how to make big girls in Czechoslovakia," said Russell. "They grow up using those no-pressure Pirelli tennis balls that are as heavy as the tires. Those balls come in a box, you know, not a can."

But soon Russell had worked up her usual lather. "I outsweat everybody on the tour. That's the one thing I'm the best at," she says. "A record 14 sweatbands in one match."

Actually, hard work on strokes and the youthful energy to run down anything are Russell's only conspicous weapons.

"I have weird vision," she says, peering through her yellow-tinted glasses. "Astigmatism in one eye. The other has what's called 'lazy eye.' It won't work in unison with the other one. Put 'em together and I'm farsighted too.

"I use one eye for backhand, the other for forehand," she deadpans."On a sunny day. I can burn a hole in a piece of paper with these lenses. I was sent home in the first grade for burning hair off peoples' arms with 'em."

That lazy eye is the only lazy part of Russell's body. If women's tennis has a John Havlicek, constantly bursting with energy on and off the court, it is Russell.

Although she has only reached one semifinal in her two pro years, she has still far exceeded the predictions made for her.

Russell has learned to be leery of big hopes. "Once before I was in the postion where I am this week. The draw was wide open and my eyes were as big as dollar signs. I was looking ahead two matches to the finals and all of a sudden they rolled me up and sent me home in a box."

Russell also has one offcourt obsession. "I'm the only philatelist on the women's pro tour," she said conspiratorially.

And that, as Cindy Harris and the door'guarding crew would say, gets the stamp of approval and a six-gun salute to boot.