Korchnoi's gamble in Game 6 may have cost him the match

Gari Kasparov of the Soviet Union, after trailing for nearly half the match, finally pulled even at the midway point and then quickly opened up a commanding lead over Viktor Korchnoi in their World Championship Candidates' semifinal match.

The favored Kasparov had been playing ''catch-up'' since Game 1 when Korchnoi , a Soviet defector now representing Switzerland, parlayed an opening advantage into an important victory. After that the competitors battled to four consecutive draws. Just when some knowledgeable players were beginning to think that Korchnoi would try to hold on and win the match by drawing the remaining games, he opened up in Game 6 and gambled to win an insignificant King Rook pawn while allowing Kasparov dangerous connected pawns on the opposite wing.

Probably Korchnoi banked on a well-posted Knight to thwart the advance of these pawns and misjudged the consequences of Kasparov's exchange sacrifice on Move 40.

Certainly it was impossible to calculate all the hair-raising possibilities of the ensuing ending prior to adjournment at Move 41, and it is unlikely that players or analysts got any sleep, since the game had to be resumed the following day. When it was, Kasparov gradually got the upper hand and eventually scored the full point, then won again in Game 7, drew Game 8, and won Game 9 to take a 51/2-31/2 lead with only three games to go.

Why did Korchnoi gamble when he could have played safely for a win, since he even enjoyed a minute advantage before the ill-fated complications? In all sports, including chess, it is axiomatic that you go with your best, with those things that have enabled you to win; and this is Korchnoi's style, which until now has proved successful against all comers except World Champion Anatoly Karpov.

Actually he might have drawn the difficult endgame, according to many analysts, including Yuri Averbakh, preeminent endgame authority. Averbakh is attending the matches as Vassily Smyslov's chief second in his match against Ribli, which stood 51/2-31/2 in Smyslov's favor with just three games remaining at this writing.

Tarrasch Defense Korchnoi Kasparov 1. P-Q4 P-Q4 2. P-QB4 P-K3 3 N-KB3 P-QB4 4. PxQP KPxP 5.P-KN3 N-QB3 6. B-N2 N-B3 7. O-O B-K2 8. B-K3 P-B5 9. N-K5 O-O 10. P-N3 PxP 11. QxP Q-N3 12. R-B1(a) QxQ 13. PxQ N-QN5 14. N-R3 P-QR3 15. B-Q2 R-N1 16. BxN BxB 17. N-Q3 B-Q3 18. N-B2 B-KN5 19. K-B1 B-KB4 20. N-B5 KR-B1 21. N-K3 B-K3 22. P-QN4 K-B1 23. R-B2 K-K2 24. K-K1 P-KR4 25. R-N2 R-B2 26. N-Q3 R-QR1 27. P-N5 P-R4 28. P-N6 R-B3 29. R-N5 P-QR5 30. NxP ch (b) NxN 31. BxN BxB 32. RxB RxP 33. RxKRP R-N6 34. K-Q2 P-QN4 35. P-R4 R-QB1 36. P-N4 P-R6 37. P-B4 R(1)-B6 38. R-Q5 K-K3 39. R-R5 P-N5 40. R-R5 RxN ch (c) 41. PxR BxP ch 42. K-K2 R-B6 43. P-N5 B-B8 44. P-R5 (d) P-N6 45. R(5)xP BxR 46. RxB P-N7 47. R-R6 ch K-B4 48. R-QN6 R-B7 ch 49. K-K3 KxP 50. P-Q5 KxP 51. K-Q4 P-N4 52. R-N8 P-N5 53. P-Q6 R-B3 54. K-K5 R-B4 ch 55. K-B6 (e) P-N6 56. RxP R-Q4 57. KxP RxP(3) 58. R-Q2 K-N5 59. PQ4 K-B4 60. K-K7 R-Q4 61. R-Q3 K-B5 62. K-K6 R-KN4 63. P-Q5 (f) R-N3 ch 64. K-K7 P-N7 65 . R-Q1 K-K4 66. P-Q6 R-K3 ch 67. K-Q7 RxP ch 68. RxR P-N8:Q 69. R-K6 ch K-B4 70. R-Q6 Q-R2 ch 71. K-Q8 K-K4 72. R-KN6 Q-R4 ch 73. K-Q7 Q-R5 ch 74. K-K7 Q-R5 ch 75. K-B8 Q-Q1 ch 76. K-B7 K-B4 77. R-KR6 Q-Q2 ch 78. Resigns (g)

A. This immediately effective Rook deployment was the purpose of White's delaying the development of his Queen's Knight.

B. This wins a pawn under dubious circumstances.

C. After this sacrifice it is clear that Black's Queenside pawns are more menacing than those of White on the King's wing.

D. To gain time later by deflecting the Black King.

E. It is not clear how Black could play for a win if White returned with 55. K-Q4.

F. 63. R-Q1 might yet have drawn.

G. A ''book'' endgame has been reached where the Rook will soon be forced to leave the protection of the King when it will be lost after a series of Queen checks. Otherwise the Black King will infiltrate to assist in a mate. This was an incredibly difficult game for both players in all its phases.

International Grandmaster Arthur Bisguier is a former US champion, has won or shared the US Open title five times, and has captured virtually every other major tournament in this country at least once during more than three decades of competition.

Next week the chess column will appear on Tuesday.