Less than 24 hours after battling tennis champion Martina Navratilova, high school senior Eleni Rossides had traded her tennis racquet for a calculus textbook and was trying to regain her balance.

Rossides' classmates at Sidwell Friends School weren't helping. In every class, they greeted her with cheers and applause. At lunch, she got a rafter-rattling midday reception in the cafeteria. The congratulations were for the 17-year-old's gutsy performance last week in the Virginia Slims tournament at George Washington University's Smith Center.

Smiling, and a bit out of breath from interrupted conversations with friends and teachers, Rossides seemed genuinely embarrassed by the excitement swirling around her.

"She just didn't talk about her match against Navratilova ," said senior John Bostwick. "She didn't seem to understand what the big deal was." Rossides and her boyfriend, John Horning, a top athlete at Sidwell, are referred to as "Mr. and Ms. Modest" by friends.

In the opening round of her first major professional tournament, Rossides, the top-ranked girl's amateur player in the mid-Atlantic region and a District resident, lost to Navratilova, 6-3, 6-2. But the teen-ager impertinently grabbed five games from the world's best.

Rossides got her shot at Navratilova after being invited to the competition as the Washington area's best amateur tennis player.

"I proved that I have a shot at doing something really big if I devote myself," said Rossides, on the day after her big match. The powerfully built 5-foot-3 senior sat for an interview in the office of principal Clint Wilkins, which had become a temporary press room because of her new-found fame.

Promising enough to consider a professional tennis career, Rossides has consciously reserved time and energy for other pursuits such as her education. She attends the prestigious private school located at 3825 Wisconsin Avenue NW, and hopes to attend Harvard or Stanford in the fall.

"I don't want to be just a tennis person," said Rossides, explaining why she never considered moving to Florida or California for the type of full-time training regime that attracts other young aspiring pros. "People who do that are probably missing so much. What do you do after 30 when you're done playing professionally ?"

Rossides, a B-plus student interested in math and literature, admitted that trying to balance sports and scholarship creates tension.

She wants to win a professional ranking by playing in more major tournaments, aware that many players, such as Chris Evert Lloyd and Tracy Austin, became stars in their late teens.

But she doesn't know how a demanding tournament schedule will mesh with making the grade as a freshman at one of the top universities she wants to attend.

"School has always come first -- except this week," said Rossides, who twirls strands of her shoulder-length brown hair through her fingers as she talks.

"When I have major tests or term papers I just don't play tennis, but it's hard to determine priorities," she said.

Rossides, who lives near upper Connecticut Avenue NW, started playing tennis at nine -- "a copy cat," she said, who ran around the court with her two older brothers.

She soon began competing, and winning, on the regional and national junior circuits, which serve as a rigorous farm system for the big leagues of American tennis.

Her father accompanies Rossides on many of her tennis travels, serving, in his words, as "chief bottle washer, shopper and laundry man."

The elder Rossides, a Washington lawyer who five years ago assisted Navratilova with her naturalization as an American, estimated that it costs between $5,000 and $10,000 annually to train and transport his daughter. Her mother is a part-time aerobics teacher.

In addition to seven hours of high school classes, Rossides is on the court for two hours most afternoons in the winter and at least four hours in the spring and summer.

Even at that pace, Rossides practices less than many of her competitors. "I would burn out playing four or five hours a day, every day," she said. "Playing less and concentrating more is more important to me."

She's capable of playing at a much higher level," said Rossides' coach, Gene Russo, head pro at the Fairfax Racquet Club. "But it will take a lot of work" -- more, perhaps, than his pupil now puts in. Russo hopes to add more offensive punch to Rossides' backcourt game, as well as improve a serve that occasionally misfired against Navratilova.

The Navratilova match, played before a hometown crowd that included many friends, provided vital inspiration. "Millions of people screaming," Rossides recalled with understandable hyperbole. "That was really neat."