After coming close to what might have been his first victory, Boris Spassky lost the eighth game of his chess match with Victor Korchnoi in Belgrade. Yugoslavia, putting himself behind by a score of 6 to 2. Korchnoi, with four victories and four draws to his credit, is now within easy reach of the 10 1/2 points he needs to win the match and challenge Anatoly Karpov next year for the world championship.

The game, which lasted 66 moves in two sessions, was the most turbulent yet in the Korchnoi-Spassky match, with two sudden reversals of fortune. After 17 moves, Korchnoi, playing black in a French Defense, had a slightly precarious position but a two-pawn material advantage. Spassky counterattacked brilliantly, and a few moves later Korchnoi was on the defensive.

At the adjournment, with Korchnoi sealing his 42d move after thinking about it for half an hour, one of Korchnoi's seconds said that he would have trouble holding a draw. On the second day of play, Spassky was able to capture two pawns, as he had threatened, but Korchnoi, with a passed pawn left, forced Spassky to exchange a knight for it and won the endgame.

Korchnoi, who defected from the Soviet Union last year, has played three matches with Russian candidates for the world championship since then and beaten them all with practically no difficulty. His match toward a match with Karpov is shaping up as the most spectacular chess achievement since Bobby Fischer's campaign for the championship in 1971 and 1972.