Jimmy Brown is 17, a junior at Largo High School in Florida who shaves when he feels like it. Less than a year ago he was ranked 408th in the world.

But yesterday, in the first round of the D.C. National Bank Classic against third-seeded Eliot Teltscher, he was unawed. In a workmanlike fashion, he unnerved and upset Teltscher, the sixth-ranked player in the world, 6-4, 6-2.

"I didn't think about who he was. I just tried to play tennis and tire him out like he was anybody," said Brown, who has been playing beyond his years lately. He defeated Jose Higueras earlier this month and lost a close three-set match to Jose-Luis Clerc.

Teltscher's difficulties were exacerbated by a dispute in the second set that all but destroyed his concentration. And Brown's underdog status--combined with Teltscher's frequent fits of temper--put yesterday afternoon's crowd firmly behind the teen-ager.

Brown's upset was one of the few unpredictable elements in the day's action. In the featured evening matches, Ivan Lendl, the tournament's top seed and fourth-ranked player in the world, had no trouble disposing of Gabriel Urpi of Spain, 6-0, 6-3. Fourth-seeded Yannick Noah, who has defeated Lendl twice this year, appeared to be on the verge of becoming the second upset victim of the day, but eventually wore down Diego Perez of Uruguay, 7-6, 0-6, 6-1.

Urpi's attempt to defeat Lendl was so hopeless that when he finally won a game, the second in the second set, he dropped his racquet and threw his arms in the air in triumph as the crowd in the main stadium gave him a rousing ovation.

Lendl won the first two games of the match without giving up a point. He played methodically with the 176th-ranked Urpi and brought out his first-string forehand only a few times. By the second set, he and most spectators had lost interest.

It was the perfect tuneup for Lendl, who has just come back from a five-week vacation of "golfing, swimming, roller skating--the things a 15-year-old kid does when he goes to Grandma's for a month, except I didn't go to Grandma's."

"I've been working hard for two weeks, but if you don't hold a racquet and move around on a court for five weeks, it's going to take awhile for it to come back," said the 22-year-old Czech.

"In another four or five days I might be the same as I was at Forest Hills," Lendl said. "I played the best tennis of my life there."

In other first-round action yesterday, Stanislov Birner, the darling of last year's Washington Star International, was eliminated by ninth-seeded Hans Gildemeister, 3-6, 6-3, 6-2.

Teltscher appeared ready to come back in the second set, but let his emotions get the best of him. With a chance to tie at three games, Teltscher began to have trouble.

First, someone in the pro-Brown crowd called, "Foot fault!" on one of Teltscher's serves. When Teltscher protested that he had not returned Brown's volley because he had heard the call, he was warned by the umpire not to delay the match.

When he continued to argue, Brown was awarded a penalty point. Teltscher persisted, and umpire Phil Adams assessed him a game penalty. Although the umpire's decision was overruled by Ken Farrar, Grand Prix officials supervisor, for being too harsh, Teltscher was undone.

"He seemed real tired. Like he was losing it--mentally and physically," Brown said. "It was a blow to me when that penalty game was taken away from me, but by then I knew I was going to break him no matter what. From then on, I had the rhythm. I just took control."

Brown went on to win the next two games and take the set.

Noah and Perez played practically even in the first set of their match. Each held serve five times and broke his opponent once to force the first set tie breaker.

Perez scored the first three points. Noah finally won a point when Perez hit a backhand into the net. Noah then scored six of the next seven points to take the tie breaker, 7-4.

After a second set lapse during which he was shut out three times, Noah reverted to form in the third set, holding his serve three times and breaking Perez twice to go ahead, 5-0.