Sean O'Neill was 9 when his coach, Chuchai Chan, made him hit 2,600 consecutive forehands. For that grueling one hour and 45 minutes of table tennis, O'Neill did not take his eye off the little white ball. Chan, a former Thai champion, wanted to teach him to mesh concentration with top physical conditioning.

The strategy has paid off. O'Neill, 18, a recent graduate of Marshall High in Falls Church, is the top-ranked amateur table tennis player in the country. Three weeks ago at the National Sports Festival in Baton Rouge, La., he defended his singles title and also swept to a gold medal in the team competition and a bronze in men's doubles. He lost just two games in 17 matches.

"I think I prepared myself pretty well for the tournament," said O'Neill, who last week played in the National Junior Olympics at Iowa City.

He has been the top-ranked junior player in the country for five years and has won 35 national and international titles. He has represented the United States at the world championships and he won the U.S. Open AAU title in June at Miami, where he beat Swedish champion Erik Lindh, the 11th-ranked player in the world. That victory was especially gratifying for O'Neill, who was severely beaten by Lindh in February.

"He kicked my butt. I had no idea what to do against him," O'Neill said. "I improved my serve and, this time, I beat him relatively easily."

O'Neill always seemed bound to take his sport beyond the walls of American basements and recreation rooms. As an 8-year-old member of the Northern Virginia Table Tennis Club, his play caught Chan's attention when his family housed Chan during an international tournament. Chan invited O'Neill to attend a training camp in Minneapolis, where he had his first taste of a strict physical regimen. After running a couple miles early in the morning and playing for hours every day, he began to take the sport seriously. Since then, he hasn't had more than a three-week layoff.

"I'm sure there have been times that he hasn't wanted to practice, but not recently," said O'Neill's mother, Kathleen. "He was at a level when he could see how much was in store for him. You have to look at it like taking music lessons. Mainly because he's done so well, he keeps his enthusiasm."

O'Neill has managed to keep that enthusiasm in a country that does not take his sport seriously. The best U.S. players go abroad to find the coaching and training techniques necessary to reach a world-class level.

When O'Neill was 11, a Swedish coach approached him at a tournament and asked if he would like to train in Stockholm. His sixth-grade teachers advised O'Neill not to pass up the experience, so he went to Stockholm, taking his schoolwork with him. For six weeks, he trained with a club there and has since been back several times. He has also traveled to China, where the class of the world is drawn from an estimated 10 million players.

Although Chan has long since returned to Thailand, leaving him without a coach, O'Neill runs every other day near his home in McLean. For almost two hours he sprints from one mailbox to the next mailbox, then jogs to the third mailbox to recover before taking off on an all-out sprint again -- a training technique known as a fartlek. On the days he doesn't run, he rides his bike. He also lifts weights, but not heavily because he does not want to inhibit flexibility and quickness. He is a wiry 6 feet 1.

"He pushed himself; he can discipline himself," said Chartchai (Hank) Teekaveerakit, the current Thai men's and junior champion.

Teekaveerakit, 18, is a protege of Chan's who has lived with the O'Neills for a year. He wanted to continue his education in the United States and Chan, aware that O'Neill needs a player of equal ability to practice with to achieve his potential, sent Teekaveerakit to Virginia.

Since there are fewer than 10,000 players in the United States and the top players are widely scattered, it is difficult for them to find adequate training partners. So O'Neill and Teekaveerakit spend hours playing, talking and analyzing their games.

"He knows as much, technically, as anyone in the U.S.," said O'Neill. "It's such an advantage to have him in my corner."

Table tennis will be an Olympic event for the first time in 1988, and although he has been accepted to George Washington University this fall, O'Neill said he might leave after one semester to train in Sweden.

"College will always be there," he said. "If I don't do table tennis now, I might not get a chance later."