Last week, Boris Spassky seemed to be doing imitations of Bobby Fischer, and it worked so well that he has won games 11, 12 and 13 of his chess match with Victor Korchnoi. Now, it's Korchnoi's move, and the stormy, self-exiled Russian grandmaster has topped Spassky's act, storming not only out of the match but out of Belgrade, where it is being played, and vowing that he will not return until the match is moved to a new location or the sometimes turbulent audience is removed.

Korchnoi is still leading by two points, 7 1/2 to 5 1/2, despite three disastrous losses in a row; he needs only three wins, six draws or a combination of the two totaling three points to defeat Spassky and win a crack at world-champion Anatoly Karpov.

But Spassky's early losing streak, which had him as much as four points behind at times, has been resoundingly ended, and the former world champion is notoriously dangerous in the late rounds of a match when he has been behind. Twice already this year, Spassky has come from behind to beat Czech grandmaster Vlastimil Hort and Hungarian grandmaster Lajos Portisch in earlier championship elimination matches. But neither had been as much as three points ahead of him.

The tide began to turn for Spassky when he stopped sitting at the playing table between moves and took up residence, during the games, in a small cubicle from which he could watch a large, electronic demonstration board set up for the spectators. Arguments over this practice delayed the match for a week (while game 10 - which Korchnoi eventually won - remained unfinished), before a compromise solution was reached.

In Belgrade, fans are speculating that the match organizers will meet Korchnoi's demand that the match be played privately, with only the two players and a referee, in order to have it continue.

In game 13, which is printed below, Korchnoi's present nervous condition can be judged from the blunder he made on the 32d move. Spassky is threatening to break through his defenses, in the diagram, perhaps with 32 . . . NxB; 33. PxN, Q-Q6ch and 34 . . . QxNP. But Korchnoi should be able to equalize with 32. RxB, RxR; 33. BxP, threatening both black rooks. His actual move may have anticipated 32 . . . BxB, which would allow 33. R-R8, Mate - but this is hardly the kind of play one would expect from a championship-level grandmaster, even under extreme time-pressure.