The decisive game of the 1966 World Championship Match. Spassky (who played many such fianchetto extravaganzas in this match, actually essays the Polish Defense (1... b5). Needing 2½ points out of the next three games, he's forced to play for a win. A draw means that he must win the next two against a man who almost never loses two in a row.
At one point, the position is repeated three times. Petrosian could have claimed the draw, but declined to do so, knowing that the only way Spassky could avoid the repetition, would be to put himself in a position where he stood clearly worse. Spassky, on his turn, also eschewed claiming the draw by repetition, accepted the challenge, and broke the repetition to try for a win. However, his courage was not rewarded. Petrosian went on to win the game, and scoring the necessary 12 points needed to retain his title. Spassky's first quest for the World Title was now over.