Magnus Carlsen’s 15-year reign as world No. 1 is under threat after he lost to Haik Martirosyan, leaving his live rating at 2809, just nine points ahead of Fabiano Caruana. If Friday’s second-game results repeat the first—Carlsen loses and Caruana wins—Caruana will overtake him. Carlsen has scored no wins in five games, while Caruana has won four.