Grandmaster Victor Korchnoi, who defected from the Soviet Union a year ago, has taken a substantial lead over a former colleague in the semifinal matches leading up to a challenge to the world chess champion. Korchnoi won his first three games against Soviet grandmaster Lev Polugaevsky and drew the fourth, taking a 3 1/2 to 1 1/2 lead in the 16-game match, which is being played in the French resort town of Evian. All he needs to do to secure a place in the challengers' final match is draw the remainder of his game.

In Geneva, Switzerland former world champion Boris Spassky, who is also living outside the U.S.S.R. but has not officially defected, is having more trouble in his match with Hungarian grand master Lajos Portisch. After four games, their score stands at 2 1/2 to 1 1/2 with one victory for Portisch and three draws.

The winners of these two matches will play one another before the end of this year to determine which one will challenge world champion Anatoly Karpov next year. Korchnoi lost to Karpov in the final challengers' match three years ago and Karpov became the champion automatically when Bobby Fischer declined to defend his little.