After a day of frantic sideline maneuvers, challenger Viktor Korchnoi's stunning comeback in the world chess championship appeared last night to be about to end in a heart-breaking final defeat.

Korchnoi, a Soviet defector now living in Switzerland, labored under the malevolent stare of a bug-eyed Soviet psychologist whose presence in the fourth row earlier in the match had sent the challenger into a rage. Korchnoi ran short of time and blundered into a position that left world champion Anatoly Karpov and his entourage of advisers and high-level Soviet officials almost certain of victory.

Korchnoi's two young seconds, both English grandmasters, indicated they could see no way for their man to recover when play resumes today. "It's hopeless," said veteran international master and chess writer Harry Golombek as he hunched over his own small chessboard in the packed press room beneath the playing hall.

"You don't have to be a grandmaster to understand the position," said former world champion Mikhail Tal, who has been advising fellow Soviet countryman Karpov in his battle with Korchnoi, a Soviet defector living in Switzerland.

Korchnoi sealed his 41st move for opening when the two men resume play today in this 32nd game of the marathon championship. Some experts thought, however, that Korchnoi might not even bother to return to the chessboard at the convention center here.

In the past two weeks Korchnoi has come back from a seemingly hopeless 5 to 2 deficit to even the score at 5 wins piece. Once or twice he has managed at the last minute to save a game that even his own seconds told reporters was lost. But most experts here insisted Karpov would get the sixth and deciding win today, ending a match that has lasted 13 weeks and included 21 draws, none of which counts under the championship scoring system.

Such a victory would bring a sigh of relief to the huge and secretive Soviet delegation here, which has grown both larger and more nervous as its champion's play has deteriorated in the last two weeks. The Soviet minister of sports, Victor Yvonin, arrived here just in time to see Karpov lose his 50th game on Friday the 13th. At that point many observers here thought Korchnoi had built up unbeatable momentum.

Karpov, playing white, ended yesterday's session with two pawns far advanced on the queenside and poised manacingly to capture Korchnoi's pieces or force them out of position. The pawns, the weakest pieces on the board, are also potentially the strongest because a pawn can become a queen after it moves to the back rank. Karpov's advantage is materially only one pawn, which he won on move 31, but the position of the pawns and the fact that they are easy to defend and advance makes his attack overwhelming.

The seeming collapse of the challenger's position last night was almost an anticlimax to a day in which the bizarre psychological maneuvers that have charaterized the match from its beginning reached new heights. Korchnoi, 47, like Karpov, is from Leningrad, but he defected to the West two years ago in protest of what he said was harrassment and neglect by the Soviet chess federation.

Moscow was putting all its hopes on Karpov, 27, to regain the world title from the brilliant but eccentric American, Bobby Fischer, who was champion from 1972-1975. When Fischer declined to defend his title in a dispute over rules, Karpov was declared champion by default. Then, to the Soviets' chagrin, Korchnoi managed to win a series of qualifying matches thought too wearing for a player his age and earned the right to meet Karpov here for what is now a $440,000 first prize.

The enmity between Korchnoi, the outspoken defector, and the massive Soviet delegation surrounding Karpov has fouled the usually sweet, clean air of this Philippine mountside resort. The two sides have quarreled over the size of the players' chairs, the flavor of Karpov's yogurt and the tint of Korchnoi's sunglasses. But yesterday the sideline feuding returned to the two most salient issues: the presence at Korchnoi's villa of two American yoga specialists recently convicted of attempted murder and the presence in the playing hall of a Soviet psychologist Korchnoi had accused of trying to hypnotize him.

The yoga experts, Steven Michael Dwyer and Victoria Shepperd, are members of the India-based Anada Marga sect and were recently convicted by a Philippine court of knifing an Indian diplomat as part of world-wide campaign by the sect to force the Indian government to release the sect's jailed founder. Both are free on bail and, replendent in bright-colored robes, have become a fixture in Korchnoi's entourage.

His remarkable comback in the last few weeks has been attributed in part to meditation exercises taught by the American couple. They have seemed to calm Korchnoi's nerves, which were badly frayed by his early defeats.

The Soviets and the Philippine organizers of the tournament have vigorously protested the American's presence, however. Yesterday, the Soviets apparently threatened to pull Karpov out of the match and the International Chess Federation jury members who oversee the championship held a lengthy meeting. It ended only when Raymond Keene, Korchnoi's chief second, hurried to a local police station and signed a statement asserting that Dwyer and Shepperd were on their way out of town.

Some Korchnoi aides revealed privately, however, that at least one of the two Americans was still in Baguio as yesterday's game began at 5 p.m. At a press conference, Soviet delegation leader Viktor Baturinsky, a lawyer with the girth and comic touch of a Nikita Khrushchev, attacked the yoga specialists with great agitation.

"At the Karpov villa, there are visitors but there are no criminals or onvicts among them." he said through an interpreter "What if 10 or 15 gangsters had come with the American basketball team (recently in Manila for a tournament); would that have been okay?"

Early in the tournament, Keene had arranged a truce with the Soviets that called for Karpov's personal psychologist, Vladmir Zoukhar, to move well to the back of the playing hall where Korchnoi could not see him. In return, Korchnoi agreed to stop wearing reflecting sunglasses that Karpov said distracted him.

Yesterday reporters Zoukhar was back in the fourth row. His unblinking stare, framed by sunken cheeks and long, lank hair, the psychologist seemed to be leaning slightly forward as if to encourage Korchnoi's stated fear that he is the target of a psychic whammy. But the challenger gave no sign during yesterday's game that he noticed Zourkhar, unlike an earlier game in which he threatened to punch the psychologist if he did not move to a back row.

In the three months the two men have played, the dollar value of their prizes, set at 700,000 Swiss francs for the winner and 400,000 for the loser, have climbed to about $440,000 and $250,000 respectively.

Technically, the funds to pay for a world championship match are raised by the national chess federation sponsoring it.