Reigning world chess champion Magnus Carlsen failed to decisively beat Russian grandmaster Sergei Karyakin and retain his crown, sending the match to a final series of tie-breakers.
Almost three weeks after the world championship began, the last scheduled game in New York ended November 29 in a draw.
The Norwegian Carlsen, 25, had been heavily favored when the series began on November 11, but Karjakin, 26, won the eighth game after seven draws. Carlsen evened the series by winning the 10th game, and neither man could break the tie in the 11th game.
Regulations stipulate that the two players will now participate in four rapid games, which the World Chess Federation says will be played "at the rate of 25 minutes per player per game, with 10 seconds added after each move."
All previous games allowed for more than three hours of play.
In the event of a draw at the end of those four games, the players will participate in two blitz games, a fast format played at the rate of five minutes per player at the start, with three seconds added after each move.
Should there still be a tie at this point, they will play up to four more sets of blitz games.
If there is still no winner, Carlsen and Karyakin will play an "Armageddon" game in which white has five minutes and black has four, but black only has to draw to win the match, according to federation rules.
The winner will be declared world champion and will take home 600,000 euros ($637,000). The loser will walk away with a consolation prize of 400,000 euros.