Halfway through the world chess championship quarter-finals, three clear leaders have emerged among the eight grandmasters competing for a chance to play against world champion Anatoly Karpov next year.

After six games in their 12-game match in Roterdam, Lajos Portisch of Hungary leads Bent Larsen of Denmark by two points. In Lucerne, Lev Polugaevsky of the U.S.S.R. is a point ahead of Henrique Mecking of Brazil, and in Revkjavik, Boris Spassky of the U.S.S.R. (currently residing in France) has a one-point lead over Vlastimil Hort of Czechoslovakia.

Still evenly balanced is the match that promises to be the most bitter struggle of all, between Tigran Petrosian of the U.S.S.R. and Vikto rKOrchnoi, who defected from the U.S.S.R. during a tournament in Holland last summer. After six games, their match in II Ciocco, a resort near Lucca, Italy, is tied with three points apiece.

Korchnoi and Petrosian were personal enemies even before Korchnoi asked for political asylum in Holland, and with the added complication of Korchnoi's present "turncoat" status in the Soviet Union, the atmosphere at their match has been below freezing; the two opponents did not speak to one another at all during the first six games of their match and the usual handshakes at the beginning and end of games have been passed over.

For a while, it was uncertain whether the match would even take place; the Soviet Chess Federation tried to have the "traitor" removed from the world championship competition entirely and threatened at first that no Russian would play against him. When the International Chess Federation made it clear that in that case Korchnoi would win by forfeit, Petrosian was allowed to play. The match began with four draws in a row, Korchnoi trying to generate attacks and Petrosian smothering them. In the fifth game, Korchnoi finally broke through to his fist victory. Two days later, in the sixth game, Korchnoi showed signs of nervousness and irritability, leading to a series of mistakes which Petrosian exploited to even the score.