As expected, champion Gary Kasparov finished off challenger Anatoly Karpov quickly and efficiently yesterday in Game 11 of the world chess championship. The win puts him ahead, for the first time in the match, with a score of 6 to 5.

The game's second session lasted only nine bri moves, played in less than 15 minutes. When Karpov tipped over his king, signaling capitulation, he was three moves away from a checkmate, but the 24-year-old champion was only two moves away from making a new queen with check.

The psychological impact of this game, which Karpov had seemed to be winning before he blundered on Move 35, was dramatic -- on the players and on the expert observers in the Teatro Lope de Vega in Seville, Spain. Experts were unanimous in labeling the error Karpov's worst ever against Kasparov and possibly the worst of his career. They found it particularly surprising because it came in the kind of quiet, technical position where Karpov is usually at his best.

"I think Karpov will always remember this 11th game as his Black Monday," Danish international master Bjarke Kristensen told Associated Press reporter David Goodman. "If I had shares in Karpov they would surely fall. His chances in the match have plummeted."

Soviet grandmaster Tamas Georgadze thought Karpov's "nerves must have got the best of him."

"Very strange -- very unusual," said Swedish grandmaster Ulf Andersen. "In one move, Karpov gave the whole game away. Shocking."

Before Karpov's blunder Monday, Dutch grandmaster Gennadi Sosonko had said, "I don't see how Kasparov is going to maneuver out of this one."

Then Karpov moved his rook when he should have moved his bishop and lost the exchange. Kasparov raised his hands to his mouth in disbelief when he saw the move. His previously downcast expression was transformed; his eyes opened wide in surprise. Then he quickly made a simple move, winning a rook for a bishop, and suppressed a grin, covering his mouth with his right hand.

A few moves later, exercising extreme caution, he pondered for 33 minutes before writing down his 41st move, sealing it in an envelope and adjourning the game. When the envelope was opened yesterday, that move turned out to be extremely efficient, giving the rook access to the heart of the white position, where it was able to destroy Karpov's prospects in a few moves.

The unplayed sequence might have gone 51. d6 (planning 52. d7 and 53. d8=Q, mate), b2. Then, if 52. d7, d1=Q, ch. Kasparov's check, giving him a one-move edge over Karpov's hypothetical checkmate, would be enough to make the difference between winning and losing. With a new queen, Kasparov could easily keep the challenger in check while driving him into a checkmate situation. Karpov could prevent the pawn from queening only by exchanging his bishop for it. Then, with no bishop to defend them, the white passed pawns could be stopped easily by the black king, while the rook would give black a winning advantage.

Karpov will now have to win two games out of the next 13, even if Kasparov does not win any more; the first player to win six games or 12 1/2 points will win the title. Until now, Karpov has used only one offensive weapon in this match, and Kasparov obviously has become familiar and comfortable with Karpov's recent handling of the Gru nfeld Indian Defense. Karpov must sharpen some new knives if he hopes to draw blood.

Grandmaster Lubomir Kavalek contributed to this report.