* Bobby Fischer, using a defense that helped him beat Boris Spassky in a key game 20 years ago, defeated his old rival again Wednesday to take a 6-3 lead in their $5 million chess rematch.

Spassky, playing white in Game 16, got into severe time trouble late in the match and resigned after Fischer's 34th move.

It was Fischer's first victory in five games since the match moved to the Yugoslav capital from the Adriatic coast resort of Sveti Stefan. Spassky had won once, and the three previous games before Wednesday ended in draws.

Spassky had opened with his usual pawn to queen 4, and Fischer responded with the Benoni Defense for the first time in the match.

Fischer used the Benoni against Spassky in Game 3 of their 1972 championship match, when he was down 2-0. Fischer won the game, his first-ever victory against Spassky. He went on to win the match and capture the world title.

On Wednesday, the opening evolved by transposition into the King's Indian Defense.

Experts watching the match initially thought Fischer's sacrifice of a knight for a pawn on the ninth move was an innovation. But Fischer, renowned for his encyclopedic memory, referred to it as "an old opening trap, 20 or 25 years or more old."

Draws do not count under special rules devised largely by Fischer. The first player to win 10 games takes the first prize of $3.35 million, and the loser gets the remainder.

The next game was scheduled for yesterday; results were not available at press time.

The International Chess Federation stripped Fischer of his world title in 1975 because he refused to defend it. He insists he is playing for the title again now, but the match is not sanctioned by the federation.

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