Challenger Victor Korchinoi, the Russian defector who had fallen far behind in the world chess championship, last night pounced on the overeager champion, Soviet protege Anatoly Karpov, and scored a much needed win.

The victory still leaves Korchnoi behind four wins to two, with a total of six wins needed for the championship, but the win is certain to help restore the challenger's confidence after a disastrous August slump. A rejuvenated Korchnoi could drag the 2-month-old, tumultuous contest into October or beyond, for the two closely matched players have already drawn 15 of the 21 games they have played.

The letest game, like much of this series, was distinguished by small mistakes rather than grand initiatives. Playing black, Karpov first launched an uncharacteristically agressive attack that bogged down in several carefully laid Korchnoi traps. By the time the two men adjourned after 5 hours and 42 moves late Tuesday, Korchnoi had pushed Karpov back and seemed in a good position to press his advantage.

Mikhail Tal, a former world champion and Karpov aide known for his ability to visualize instantly several moves in advance, said he thought Karpov was doomed after the first night. Last night, after less than two more hours of play and 18 more moves, Karpov resigned, for he was faced with the impossible choice between losing a vital rok or bishop or letting Korchnoi promote a pawn to an all-powerful queen.

Korchnoi, obviously delighted, left the convention center where the games have been played in this mountain resort for his private villa to eat and begin planning strategy for Thursday's scheduled 22nd game. "I feel that Karpov is getting tired," Korchnoi said to reporters before leaving. "He's winning, but he needs some strength, some force, to finish." The reclusive champion left quickly without stopping to talk to reporters.

The clash between the 47-year-old Soviet defector and the 27-year-old Soviet loyalist has produced a series of bitter controversies, ranging from how high Korchnoi's chair could be to whether Karpov could station a psychologist in the playing hall whose bugeyed shares made Korchnoi think he was being hynotized. Korchnoi retaliated by invitin in new-found friends from an Indian religious sect to counter the bad Soviet vibrations. This created more controversy when the two American-born members of the sect were revealed to be out on bail on an attempted murder conviction.

Up to now, the bickering had seemed to distract only Korchnoi. Karpov continued his cool, methodical play on the chess board and his strict schedule of tennis, rest and strategy sessions between the three-times-a-week games.

Karpov has been an agressive player in blowing over weaker opponents since he won the championship by default in 1975 when American Bobby Fischer refused to defend the title.

But faced with the dangerous counterattack skills of Korchnoi, Karpov, in this series, has been more careful. "Karpov is like a tennis player just trying to get the ball back," said English grandmaster Michael Stean, who helps the challenger plot strategy against Karpov. "He's playing baseline chess. He won't come to net."

Several grandmasters watching last night agreed this was the most aggressive game Karpov had played in the series. "Maybe, he's become impatient, and wants to know Victor out of the box," one said. Korchnoi, baptized a Roman Catholic during his boyhood in Leningrad, allueded to the resulting Karpov blunders with a biblical quote: "Forgive them Lord, for they know not what they do."

It was suggested early in the series that Korchnoi, although playing much better than anyone else in the world his age, would eventually tire in a long conetst with the much younger Karpor. Chess players don't move much, but physical weakness and age can still produce fatigue near the end of a five-or six-hour match, which can lead to bad moves and missed opportunities.

Kochnoi, however, has appeared to be in excellent shape. A 33-year-old reporter, in fairly good condition, was left far behind by the challenger during a brisk mile run around a muddy local golf course. Korchnoi's supporters have pointed out that in his last series against Karpov, a 1974 championship qualifying match, the slim younger player took an early lead but began to slip and just barely held onto a razor-thin edge of three wins to two.

Since Korchnoi took a week's rest in late August and early September to recover from a disastrous night to which he lost two adjourned games in the space of 90 minutes, he has appeared refreshed and stronger, while Karpov's play has become erratic.

Sunday afternoon, a room for kibitzers set up in the convention center basement broke into a raucous buzz when Karpov unsealed a move to resume the adjourned 20th game. Korchnoi's aides has virtually conceded the game to Karvpov, based on his dominant position, but the unsealed move revealed that the champion had somehow missed a clear opportunity to ensure the win. "Absurd," [WORD ILLEGIBLE] a blunder." came the outcry in fractured English and curt Russian from the grandmates and chess writers assembled in the basement where their talk wouldn't disturn the players.

Some have blamed the blunders by both Karpov and Korchnoi on the battles over chairs and psychologists and Indian sects, [WORD ILLEGIBLE] Stean argues that the sometimes messy play was foreordained by the styles of the two contestants. "Both of them play destructive chess. Their specialty is laying back and destroying the other position, forcing mistakes," he said.

In the rarefied air of world class chess, the intellectual powers of the players seem even cooler and clearer than the air of this resort, built for Americn colonial administrators who once sought relief from Manila's heat. What is called a mistake at this level of play is no more than a small wispy cloud on a distant horizon, impossible to detect unless one has lived and played up there a long time.

Karpov and Korchnoi suffer by comparison from the brilliance of the last world championship in 1972, when Fischer overwhelmed the then-champion, Boris Spassky, with a virtuoso display of what Stean calls, "constructive chess."

That contest in Reyjavik, Iceland, lasted 21 games, a number this series has just passed with no immediate end in sight. That does not bother the habitues of the convention center basement, an odd assortment of Russians, Yugoslavs, Britons, Argentinians and Americans. To them the slight movements of bits of wood on a checkered board upstairs, transferred by couriers to a much bigger board downstairs in the basement, shed blood and splinter bone, produce pain or ecstasy.

Just how much the strain has affected Karpov, surrounded by Soviet officials who ache to defeat a traitor like Korchnoi, may be revealed Thursday. Korchnoi has used up his three postponements allotted for the first 24 games, but Karpov has one left and he may feel like a break right now.