A 19-year-old Potomac youth yesterday won the world junior chess championship in Groningen, Holland, beating 54 other national champions from around the world with a come-from-behind victory in his final game.

Mark Diesen, 8617 Red Coat La., a University of Maryland freshman and a 1975 graduate of Montgomery County's Winston Churchill High School, forced Marcel Sisniega of Mexico to resign after 42 moves in the concluding game in the annual tournament.

With yesterday's victory Diesen earned for himself the title of International Master, just one step below the chess world's coveted grand master status - a ranking he hopes to achieve within the next two years.

"I'm going to rest here for a few days, and then return hom on the seventh (of January), I wouldn't want to miss the Super Bowl," Diesen said in a telephone interview yesterday.

Diesen had to defeat Sisniega, the national junior champion of Mexico for his victory. "He played accurately at first, but I outplayed him in the end. Once I took his queen by force it was totally hopeless for him," Diesen said from his room in the Euro Motel where the 13-round championship was held.

Diesen, who whose father taught him the rudimentary moves of the game nine years ago, is currently the U.S. Junior Champion, and s ranked 23d among all Americans players. He also is ranked among the top 300 players world-wide, and his standing in both areas is expected to improve as a result of yesterday's victory, according to his father, Carl Diesen.

"Chess is basically a meaningless, abstract competition, like football or any other sport," said the new world junior champion, a title open to anyone under 20 years of age. "What does it matter if you move wooden pieces around on a board, or run up and down a field with a ball? It's the creative satisfaction that matters, it's using your talents in your particular field. Mine just happens to be chess," he said.

Diesen used his talents to defeat the French Defense, a protracted series of defens maneuvers that he had predicted Tuesday night that Sisniega would use. Although he acknowledge making some "tactical" mistakes early in the game, he soon overcame them and went on to defeat the Mexican junior champion.

Chess is a combination of "art, science, and sport" for the University of Maryland freshman, who took one semester of computer science classes before dropping out in June to concentrate all his energies on the tournament.

"It's art when you think of a new idea, discover a new move or relationship between the pieces," he said. "It's science when you study a position or opening or attack and then know what to do when you encounter it in a game. And it's a sport because it's always competive," Diesen said.

Diesen and his "second" - or coach - Lubosh Kavalek, a chess master who lives in Reston, were up late last night studying both the French and the Sicilian Defenses, one of which they believed Sisniega would use.

"You prepare for a chess game the way you prepare for any game," Diesen said. "You study you opponents' moves, and then you concentrate all your energies on winning. I was nervous yesterday morning, sure I ate a little breakfast, but then everything was ok," he said.

Because of intricacies of tournament scoring, Diesen needed only a tie in the match to win the title. "I wanted to keep the draw (tie) in hand, so I didn't want to press him. I didn't do anything too aggressive. But once he started to make a few errors I moved in," he said.

The match lasted five hours and 10 minutes and ended shortly before 10 a.m. Washington time, he said.

For the record, Diesen played the white pieces in the championship round. His first move was the classic P-K4 - the pawn moved to the fourth square in front of his king. Then the ultimately successful battle was joined.

The champion's mother, Marge Diesen, recalled that her son's life at home revolved around the family's numerous chess sets and books detailing famous matches. "He gets up has his breakfast, and then does some chess'" meaning that he reads chess books and magazines, replays games himself "and then tries to innovate on what the masters' have done," she said.

Mrs. Diesen said he was the Wisconsin state women's chess champion in the 1950s, having been taught the game in college by her then-future husband, Carl Diesen.

Her son's interests outside of chess include playing tennis and rooting for the Red-skins. "Several weeks ago he called us from Holland and said, 'How are the Redskins doing?' I told him. 'Never mind about the Redskins. How are you doing?'" Diesen then told her he had won his most recent match.

His father a GS 17 computer specialist at the Geological Survey in Reston ard a former nationality ranked chess player, said. "Mark asked to learn the game when he was 10. Then he just took off. It's been a long time since we could teach him anything" about chess.

As for his future, Mark thinks he might like to pursue a career as a professional chess player. "But for that you've got to write books and articles. I'm going to start think about that," he said, "as soon as the Super Bowl is over."