Game 21 of the world chess championship, played to a 28-move draw yesterday in Seville, Spain, was partly a reminiscence of bygone games, partly a quick peek at new ideas in a much-explored opening. But its chief significance was that it leaves challenger Anatoly Karpov only three more chances (one with the white pieces) to take a desperately needed full point from defending champion Gary Kasparov.

The match score is tied at 10 1/2 points with each player having won three games and drawn 15. Kasparov keeps his title if the match reaches a 12-12 tie, so the pressure on Karpov is mounting steadily with each draw. Former champion Vassily Smyslov, commenting on the game for spectators in Seville, predicted the winner of the next game will be the winner of the match.

Former champion Boris Spassky, back in Seville after a trip to the United States, where he lectured at the American Open tournament, said that "both players are exhausted and looking for the end of the suffering." Acquaintances of Karpov say that while he wants to regain his lost title, a 12-12 tie would leave him with the consolation that at least he had not lost the match. He seems to be optimistic about achieving at least that much.

The opening was again the Gru nfeld Defense, Kasparov's chief resource with black for most of the match. Karpov returned to the sharp Russian Variation, which begins with 5. Qb3. The challenger had a small trick up his sleeve, with his 14. Bf4. In Game 15, he had wasted time at this point with 14. h3, which gave Kasparov a chance to launch a queenside pawn advance: 14. ... a6, followed by 15. ... b5.

Kasparov, in a difficult position after Karpov's 16. Qb3, spent 18 minutes in thought. Then, after 18 moves, he produced a little trick of his own, echoing Game 15 in the way he used the square d3 for his knight. Strangely enough, Karpov helped him.

Karpov used almost half an hour on his 19th move; then, instead of playing 19. Nb5, he kicked black's knight into the powerful square d3. It was impossible to capture the knight with 20. Rxd3 because of 20. ... c4, winning the exchange. Many experts in Seville thought Karpov's 19th move was a blunder, and there was lively discussion of a fact well known among the top players: It is not uncommon to make a mistake after spending a long time thinking about a single move.

But was the move a blunder? The continuation of the game shows that, although black has some advantage, he cannot improve his position after his 22nd move.

The game moved into a state of equilibrium. Black was unable to play 23. ... Nxb2; 24. Qxb2, Na4, because white can win with 25. Qb5. Instead, Kasparov chose 23. ... f5. The move weakened his kingside; Karpov was able to sacrifice the exchange and, having a supported, strong, passed d-pawn, he had definite compensation for his small material loss. Kasparov, sensing this, started to hit white's queen, and the players decided to repeat the moves and call it a draw. Grandmaster Lubomir Kavalek contributed to this report