Defending champion Gary Kasparov won Game 20 of the world chess championship match yesterday in Lyon, France. The match score is now 11-9 in Kasparov's favor, an almost overwhelming advantage. To keep his title, he needs only to draw two of the remaining four games in the match, which is limited to 24 games. To recapture the title he lost to Kasparov in 1985, challenger Anatoly Karpov must win three of these four games after winning only two of the first 20.

Karpov resigned on the 41st move (and could honorably have resigned earlier) after playing a complex, violent variation of the Ruy Lopez, his favorite defense with black against a king's pawn opening. He had decided to change his variation of this defense after losing with it in Game 18, and this time he returned to ideas of his coach Igor Zaitsev, who has deeply researched the variation starting with 9. . . . Bb7 and 10. . . . Re8. Karpov used this variation earlier in the match, reaching a draw and missing a possible win after a tense struggle.

This time, however, Kasparov tried a variation on Karpov's variation; instead of 18. exf5, he played 18. Rae3, a move that was featured in the final candidates' match, Timman-Karpov, earlier this year. In that game, Karpov tried to block the position with 18. . . . f4. This time, with disastrous results, he decided to keep the tension in the center. Karpov began to undermine the advanced white pawns on e4 and d5, but Kasparov mobilized his pieces so quickly on the kingside that the elimination of center pawns made his attack easier.

When Karpov broke the center tension after 22. Bb2, all the lines to black's king were opened, and with 25. Ng4, Kasparov was ready to launch his final attack. He offered a knight sacrifice with his next move, and from there on the attack almost played itself. On move 29, Karpov had to give up a rook, and after 33. Re8, the game was virtually over. Karpov continued to play on stubbornness or sheer momentum, allowing Kasparov to crown his strategy with some beautiful tactical moves.

In the final position, Karpov can capture the white bishop, but only at the expense of having his advanced passed pawn captured and going into a hopeless endgame.

After the game, the players stayed on stage for several minutes analyzing various possibilities. Kasparov had a rare word of praise for his opponent, calling him "a great fighter."

"I'm very satisfied," he added. "I finally won a game in my old style."

Game 21 is scheduled to start on Monday, with Karpov playing white. The first player to reach 12 1/2 points takes the title and a $1.7 million first prize. In case of a 12-12 tie, Kasparov keeps his title and the $3 million pot is divided equally.

Kavalek is an international grandmaster; McLellan is a Washington Post staff writer.