Viktor Korchnoi, the Russian chess grandmother who defected to the West last year, claims that his opponents Boris Spassky is distracted by the forlorn-looking red Soviet flag at his side.

Expelled from the Soviet Chess Federation and stripped of all Soviet sporting honors, Korchnoi, 46, is playing the most important match of his long chess career, without a flag. The winner of the 20-game series now under way in a central Belgrade theater will win $16,000. But more important, he will challenge the Soviet Union's latest chess idol, Anatoly Karpov, for the world championship next year.

According to Korchnoi, Spassky dislikes being put in the position of having to defend Soviet prestige against a man officially accused of "morbid self-esteem" and "betraying the motherland." Says Korchnoi with an air of triumph: "The other day, I caught him trying to push his flag into the middle of the table. I went over to the controller and complained - and the flag was put back where it belonged, next to Spassky.

In the emotion-charged atmosphere of world-class chess, such pyschological victories off the board are almost as important as the capture of pieces on it. They have helped give Korchnoi an almost invincible lead. But Spassky, at 42, a former world and Soviet champion, has won game 11 and begun the almost impossible job of trying to win the match.

A small, puckish figure with a mischievous grin, Korchnoi is within sight of fulfilling his dream of playing the 26-year-old Karpov - who became world chess champion by default in 1975 when the temperamental Amercian genius Bobby Fischer refused to defend the title. For Korchnoi, such a match would offer the chance of sweet revenge. He maintins that he was punished for putting up too much resistance against Karpov, the official favorite, in the last candidates' tournament for the world championship.

"I want to show to everyone in the world that the Soviet Union is an oppressive society and that those people who leave fof the West can play chess better than those who stay in the Soviet Union. I feel free now. I feel that if I were to play Karpov now, I could come to his throat and seriously damage it," he says in his slightly idio-syncrathic English.

A world championship match between Korchnoi and Karpov, which now seems increasingly likely, would be a severe embarrassment for the Soviet Union - probabaly even more embarrassing than the celebrated Fischer-Spassky contest in Reykjavik in 1972. Most Soviet emigres can safely be ignored as soon as they leave their homeland, but not a chess player on top of his form.

As korchnoi remark in his recently published autobiography "Chess Is My Life" "In the Soviet Union I enjoyed a degree of perfectly official popularity that neither Solzhenitszn nor Sakhaarov could boast of nor even Rostropovich or Barshai, public figures who are much better known in the West than I am. I was seen on the television screens by tens of millions of people. I was greeted, and my speeches listened to by hundreds of thousands. For dozens years the papers talked about me - Stalin, Malenkov, Khrushchev gave way one to another, but my name did not disappear from the press."

A boastful statement perhaps, but also an accurate one in a country where chess is a national past time and layed avidly in parks, restaurants and on streetcorners.

Korchnol sees Spassky, a longtime friend and rival, principally as an obstacle to his match with Karpov. Describing Spassky as a "one-legged dissident" because of his marriage to a French girl he claims they had an unwritten agreement to support each other when they were both out of favor with the Soviet Federation.

"But the Russians are now supporting Spassky as the lesser of two evils. For the second time in his life - the first time was against Fischer - he has been called to defend the Soviet frontiers. And both times he is losing," says Korchnoi in an interview in his hotel room, surrounded by unfinished chess games.

aunts apart, relations between the two men appear cordial enough when they meet in the cavernous auditoruim of Belgrade's Trade Union Theatre which is the venue for their matches.

Spassky gingerly shakes Korchnoi's left hand (his right hand was hurt in a car accident just before the beginning of their match last month) and they briskly set about their business.

There is a swift round of applause from the expectant crowd as Korchnoi, playing white, moves immediately P-QB4, the English Opening which is his favorite in this match. Spassky has already decided on his defense and the opening 10 moves take less than 10 minutes times by the clock which the players switch off and on as they move.

Play then settles down into the middle game" - the battle of wits, strategy, and emotions with each player struggling to gain some slight advantage. The moves are displayed on a giant chessboard at the back of the stage and flashed onto electronic boards around the auditorium. The 2,000-strong audience seems as absorbed in the fame as the players themselves, sitting patiently even when Spassky ponders a record 68 minutes on a crucial move. (There is a buzz of excitement when he finally makes up his mind.)

Outside the hall, there is another electronic scoreboard. A Yugoslav grandmaster explains the significance of each move to a large crowd of enthusiasts. Young girls wearing "Spassky-Korchnoi" T-shirts sell badges and programs. Book stalls stacked high with chess literature ("Play Like a Grandmaster" and "Chess Is Chess" are typical titles) do brisk business.

Many fans follow the game on pocket chess sets - arguing with their neighbors over possible permutations. "Don't you see, by sacrificing that pawn Spassky controls the bishop's file and puts pressure on Korchnoi's rook. If Korchnoi goes there, he can go here," reasons a Spassky supporter.His friend disagrees and their discussion becomes more heated.

The audience is a mixture of young and old which, but for the striking absence of women, could almost be a cross-section of Yugoslav society. Students free from school mingle with veterans who look as though they learned their chess in between fights with the Germans in World War II. Workers just off their shift sit next to chess fanatics (some dressed in checked shirts and checked bow ties) who follow tournaments from one country to another.

Haunting the match like ghost who can't be exorcised is the mystical personality of Bobby Fischer who lives as a semi-recluse in the United States after being stripped of the world title in 1975. Both Korchnoi and Spassky pay tribute to his brilliance as a chess-player and express the hope of playing against him again. He is alternately reported to have retired from chess for good, and to be preparing a comeback, by journalists who claim to have spoken with him at one of his hideouts.

Two specially lit boxes by the side of the hall are reserved for the friends and aides of the two opponents. Korchnoi's regular entourage concists of a Dutch lady working in Switzerland who sports Amnesty International badge plus two of Britain's rising young chess stars - Raymond Keane and Michael Stean.

The rival box contains Spassky's wife, Marina, and his second, Soviet grandmaster Igor Bondarevsky.

Apart from negotiating with the enemy, it is the second's job to analyze games adjourned overnight. While Korchnoi and Spassky are creative geniuses whose strength lies in their ability to think out new lines of play, their seconds are "chess computers" - chosen because of their detailed knowledge of every important game played over the last 50 years.

It is an important task. Korchnoi attributes his advantage in the present match to superior preparation and training. "Spassky is lazy. He thinks he is so clever that he doesn't need theory, that he can just come to the chess board and invent new theories as he goes along. But chess is a serious occupation - and it requires serious study."

Spassky has been described as "the Pushkin of chess" - because of his controlled emotions and methodical, harmonious style of play. Korchnoi, on the other hand, is known in Yugoslavia as "Strashni Viktor" (Viktor the Terrible) because of his fighting spirit and his ability to exploit mercilessly and the slightest mistake of his opponent.

Support for the two is fairly evenly divided among Yugoslavs. As in Reykjavik, when he was subjected to lengthy melodramatics from Fischer, Spassky captures the sympathy vote.He is cool, obviously decent, and his haid curls modishly round his ears, Korchnoi is high-strung, aggressive - and also happens to be winning. At least he has been up to game 11.