In July of 1985, the FIDE Executive council met in Tunisia to determine the format of the match. They decided that due to problems with the unlimited match format, the new match
should return to the old system: the winner was to be the player who scored the best of 24 games, and in the case of a tie the reigning champion Karpov would retain the title. The rematch clause, which granted a defeated champion an automatic right of rematch, was in effect–however, the council decided that it would be used for this cycle only and then abandonded. This meant that for Kasparov to secure the championship, he would have to defeat Karpov in 1985 and then hold him to at least a draw in a 1986 rematch.
The match began at Tchaikovsky Hall in Moscow on September 3, 1985. It was a closely contested battle, with Kasparov pulling ahead in the the 16th game in a positional masterpiece. In the last game, Kasparov sought counterplay in the Sicilian Defense, and sacrificed a pawn on the 26th move and then another on the 31st move, to obtain activity in a complicated position. After a blunder by Karpov, a simple tactical sequence brought the house down.
On November 9, 1985, at the age of 22, Garry Kimovich Kasparov became the 13th World Champion.