LYON, FRANCE, DEC. 8 -- Undaunted by his loss in Game 17 Wednesday, world chess champion Gary Kasparov came roaring back in winning form today for Game 18 of the World Chess Championship. After 26 moves of a Ruy Lopez opening (challenger Anatoly Karpov's favorite defense against a King's Pawn opening), Kasparov had a technically won game with a well-supported queenside passed pawn marching toward the eighth rank.

Karpov continued to resist, however, first with a counterattack on the passed b-pawn and then with an attack on the white king's position. Both attacks were repulsed. When Kasparov wrote down his 41st move, to be placed in a sealed envelope and opened at the game's resumption today, Karpov was attempting to block the passed pawn and mounting an attack on white's e-pawn, but experts observing the game were unanimous in the opinion that Kasparov would win.

Kasparov also seemed to be winning off the chessboard in his prolonged and bitter campaign against the International Chess Federation (FIDE) and its recently reelected president, Florencio Campomanes. FIDE has controlled the World Chess Championship since the end of World War II, but with the Grandmasters Association (GMA), an organization of the world's top professional players founded largely through Kasparov's efforts, that control is being weakened.

At a press conference here, the GMA board of directors said the players have decided to take control of their own destinies and organize the next candidates' elimination cycle and the 1993 world championship match. Elimination matches for the 1993 championship had been scheduled to start in Djakarta, Indonesia, next Jan. 23, but Campomanes has just announced that the organizers in Djakarta are unable to meet this deadline.

Game 18 followed the moves of Game 12 through Kasparov's 12. Na3, but then Karpov surprised the champion with 13. . . . Nb6. Kasparov spent 44 minutes thinking about his 14th move, but then he made a good choice, keeping his pawn center intact at the cost of breaking up his pair of bishops.

After 21. Qc4 (by which time Kasparov had achieved a positional advantage), it was Karpov's turn to spend a long time -- more than an hour -- considering his next move. In spite of the time invested, he did not choose the best move; he should have retaken the pawn with 21. . . . Bxb2; 22. Ra2, Bc8; 23. Nb5, Qf6. This would have left him a pawn down after 25. Nxc7, but white would not have a passed pawn on the queenside.

After his attempt to storm the kingside was repulsed in moves 31-38, Karpov drew his pieces back into essentially defensive positions, but at adjournment Kasparov's advantage looked decisive. This impression would be checked by teams of experts overnight, and if the position is in fact hopeless Karpov may decide to resign without resuming play today.

Until this game is decided, the match score stands even with 8 1/2 points for each player. Each has won two of the first 17 games and played 13 draws.

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