Victor Korchnoi lost his chance for a middle-game victory, but retained a alight endgame advantage in his fifth game against world chess champion Anatoly Karpov. Korchnoi sealed his 92nd move in the game's second session with a bishop and a pawn against karpov's two pawns, facing a very subtle endgame challenge.

Most experts thought a draw was likely, although they would not rule out Korchnoi's winning chances entirely. "White can't lose the game but black can," said American grandmaster Robert Byrne.

"This is a ridiculous game," said English grandmaster Raymond Keene, one of Korchnoi's seconds. "Korchnoi is trying to drive Karpov crazy and Karpov is trying to stay alive."

Actually, the curious maneuvers in the late moves of the game below are part of a very rate endgame position, but one that is not wholly unknown. It has been thoroughly analyzed in only one book, by Soviet grandmaster Yuri Averbakh, which presumably will be consulted by both sides during the second adjournament. Karpov may be familiar with this study already; at least his moves so far have been in line with its ideas.

In the adjourned position, Korchnoi's problem is not much the blockade of his pawn as the fact that it is on the rook file. One file over, and it would be headed toward queening on a square covered by the white bishop. The pawn can be moved over to that file only by forcing Karpov to play . . . P-N5, and that can be done only be freezing the black king in a position where it is unable to move.

The question being analyzed by Korchnoi and his assistants, which will be answered when the game is resumed on Sunday, is whether Karpov's king can be frozen into such a position (which chessplayers call zugzwang). According to Averbakh, white can force a win if he can drive the black king into the lower left corner of the board (the triangle formed by the squares a6, c3 and a8 or QR6, QB8 and QR8).

The diagram shows a position of the kind Korchnoi will try to reach. At this point, white can play 1. K-N6 and win: 1. K-N6, K-N1; 2. B-B7ch, K-R1; 3. K-R6, and the white king is frozen in place. (The same is true after 2 . . . . K-B1; 3. K-B6.)

Once the black king is immobilized, the rest is easy: 3 . . . . P-N5 (forced); 4. PxP, P-R6; 5. B-K5, P-R7; 6. P-N5, P-R8-Qch; 7. BxQ, K-N1; 8. P-N6, and the rest is routine.

It would be quite easy, in the adjourned position, for Korchnoi to capture both of Karpov's pawns with his king, enjoy the apparently overwhelming advantage of bishop and pawn, and still come out with only a draw. This is because his bishop is the wrong color to help a pawn on the rook file.

In this situation, the black king can blockade the pawn by shuttling back and forth between his QR1 and QN1 squares. White cannot prevent this shuttling without making it impossible for black to move, in which case the game is a stalemate.