Last summer, when he beat Arthur Ashe in the French Open tennis championships, Ivan Lendl thrust his arms into the air and leaped what seemed to be four feet off the ground. It was his first victory over one of the world's top 10 players, a crucial psychological breakthrough. Someone asked him if this victory would be regarded as important news back home in Czechoslovakia. "I don't know if it's important for the people in my country," he said, a smile creasing the face that is always a sober portrait of concentration on the court, "but it is very important for me."

When Vitas Gerulaitis drubbed him, 6-2, 6-1, 6-3, two days later, Lendl again came off the red clay stadium court smiling. Someone asked him why. "I played my best shots, and he got to them so easy there was no chance for me," he said. "I knew before the match that he was very fast, but I didn't believe anybody was that fast. I hit many shots that against other players would be winners, but he was always winning the points."

There is a quotation from Kipling which hangs above the entrance to the Centre Court at Wimbledon. It says: "If you can meet triumph and disaster, and treat those two impostors just the same . . ." A tricky task, that, but Ivan Lendl seems better equipped emotionally than most young tennis pros to do it.

The French Open provided a valuable, Kiplingesque lesson. By beating Ashe, Lendl proved to himself that he was good and getting better. Gerulaitis showed him that he still had a long way to go. Lendl accepted both lessons with an engaging smile, keeping an even keel.

And so it was that he arrived here for the $125,000 Volvo Classic at George Washington University's Smith Center as the No. 7 seed and perhaps the brightest newcomer in the 32-man field for those aficionados who like to get an early glimpse of a blossoming talent. Last night he was impressive in thrashing Billy Martin -- the dominant American junior of a few seasons ago -- 6-2, 6-1.

From Paris, Lendl went on to have a very good year for a 19-year-old. He beat two more players ranked in the top 10: Eddie Dibbs at Vienna, Jose Higueras at Buenos Aires. He reached the finals of two small Grand Prix tournaments, the semifinals of three others. By the end of the year, he was ranked No. 22 in the world, according to the computer of the Association of Tennis Professionals. Tennis Magazine named him the men's rookie of the year in pro tennis.

Lendl -- lanky and athletic at 6 feet 2 and 170 pounds, given to taking staircases four steps at a time and casually vaulting over fences and bannisters -- was voted the best junior tennis player in the world in 1978. There was little doubt about that, since he had swept the Italian, French and Wimbledon junior titles. But while most of his contemporaries were combining junior tournaments with a liberal education in the rough-and-tumble of the men's circuit, Lendl seldom played above his age group. He thinks now that may have been a mistake.

"I think from the beginning, I played too much in the juniors. I should have stopped earlier," he said yesterday, before playing Martin.

"I was winning too easily in the juniors, and I forgot there were players who could beat me. I was surprised if somebody played great shots all the time, because in the juniors I always had easy matches until the semifinals, even if I played badly."

Now Lendl, who will celebrate his 20th birthday on Friday, is not surprised when a Bjorn Borg or a Jimmy Conners, a Gerulaitis or a Roscoe Tanner, rifles bullets at him throughout a match. He expects it. It inspires rather than awes him.

"After Geerulaitis in Paris, I played Borg twice, Conners twice, Tanner. It helps you to play the top five guys, sure. As many times as you play them it helps you," he said. "They have something that is better than the other players. But you get used to that, and then you start beating them if you are good enough.

Is that to say that the initial impression that they are supermen -- that nobody can be that fast! -- disappears, and they come to appear as mortals across the net?

"Yes, that's right, that's right," said Lendl, a bright and pleasant lad whose English is self-taught but creditable. That appealing smile lit up what can otherwise be a terribly dour countenance. Even his tiny wisp of a mustache seemed to smile. "That was what I wanted to say, but maybe I said it bad."

Lendl comes from solid tennis stock. He is the only child of Jiri and Olga Lendl, excellent players both, and picked up a racket at such an early age that he cannot remember a time when he did not play tennis.

His father -- a lawyer in Ostrava, a heavy industry city of 300,000 residents, about 300 miles northeast of Prague -- was ranked among the top 15 players in Czechoslovakia. His mother was ranked No. 2 in women's singles and briefly traveled internationally, though she could not beat nemesis Vlasta Vopickova -- the elder sister of 1973 Wimbledon champion Jan Kodes, now Lendl's teammate on the Czechoslovakian Davis Cup team.

Ivan never had any formal instruction, but he played with other children and with his parents at the best of tennis clubs in Ostrava. At the age of 8, he entered his first tournament. His forehand was his weakest shot, but he has built it into an explosive weapon. Contrary to the vogue of the day, he gave up his early two-fisted backhand and learned a one-handed top spin stroke.

For a country with barely more than 15 million people, Czechoslovakia has a considerable tennis heritage.

Karel Kozeluh never played in the world's major championships (which were then amateur) because he was a professional ballboy at age 6 and a well-known coach by the time he was 12, but he was considered one of the best players of the 1920s. Jaroslav Drobny (1945), Kodes (1973) and Martina Navratilova (1978-79) have been Wimbledon champs. Vera Sukova, who now coaches an extremely successful group of young women players, was Wimbledon runner-up in 1962. Czechoslovakian women won the Federation Cup in 1975, and the men reached the final of the Davis that year.

There has been pro tennis of sorts in what is now Czechoslovakia since 1879, when a Duke Kinski hosted a modest prize money tournament at his castle in Bohemia. Later the same year, a championship was held at a castle called Bon Repos, with the prize a barrel or Rhine wine -- presumably guaranteed to elevate the winner to a royal highness if he wasn't one already. Less intoxicating was the first inter-city competition, between Zbraslov and Kakovnik, for which the grand prize was a large cucumber.

Lendl is not much interested in history, though.

"I think the game today is much more difficult than it was before," he said. "Some people keep saying that the players 30 or 40 years ago would beat the top players today, but I don't think so. I don't think they would beat the average guys.

"It makes me mad when people say that. I don't think players from the 1930s could beat Borg. They didn't have top spin, they didn't have so much speed. They just played the ball back and didn't go for winners."

Lendl goes for winners. Always. He is a flashy player who earned $77,401 in prize money last year. He keeps 80 percent of his earnings and gives 20 percent to his government.

"At the beginning, I thought the Grand Prix tournaments were more difficult than I had expected, but after the first year, it is looking easier than I expected," he said. "Maybe the reason is that I made a little bit of progress."

Full speed ahead, but with an even keel.