Victor Korchnoi defeated Soviet champion Anatoly Karpov yesterday in a five-hour match game to even the score in their title series in the Philippines at one win each. Karpov weakened seriously in the middle game and then blundered decisively.

Off-the-board action continued with Karpov first issuing a statement questioning chief arbiter Lothar Schmid's rulings on the seating of a Soviet parapsychologist. Then, Karpov met with Schmid and made amends with a public apology.

Karpov's final statement says in part that, "the world champion retains his confidence in Mr. Schmid's objectivity and impartiality as chief arbiter."

Such remonstrations are not entirely a novelty for Schmid. He was also chief arbiter for the Fischer - Spassky match in 1972.

The parapsychologist was restricted to the seventh row of the audience for the remainder of the match. His practice of staring at Korchnoi throughout the match games has been the source of numerous protests.

In the game itself, the 11th in the match, Karpov played the opening lackadaisically and allowed Korchnoi to gain space with his center pawns. By move 20 Korchnoi had gained the two bishops and an attack.

Karpov's 22 . . . N-N2? left him with a passive position and no counterplay. Then, black's 25 . . . P-N? was a tactical mistake, missing 27. Q-QB1! which defends the bishop at king-three and sets at winning pin on the queen-bishop file. Black was forced to retreat with 25 . . . R(N)-N1, but white would have retained a free hand on the kingside.

Korchnoi exploited the pin to win an exchange (bishop-for-rook) and piled up his major pieces for an additional breakthrough. Karpov played on with the lost position and at last resigned at adjournament.