Game 6 of the Carlsen–Nepomniachtchi world championship match was one for the ages. After draws in the first five games, the world champion broke the deadlock with a 136-move victory — the longest in world championship history. It lasted almost eight hours, and Nepomniachtchi made the final mistake in an endgame with a lone queen against rook, knight and two pawns.

In a balanced middlegame, ‘Nepo’ took a risky decision at move 25, offering an exchange of two rooks for Carlsen’s queen, creating winning chances for both sides.

Magnus Carlsen–Ian Nepomniachtchi

World Championship Game 6, Dubai

25… Rac8 26 Qxc8 Rxc8 27 Rxc8 Qd5 Attacking the pawn on b3. It’s important that 28 Nf4 Bxf2+ wins. 28 b4 a4 29 e3 Be5 30 h4 h5 31 Kh2 Bb2 32 Rc5 Qd6 33 Rd1 Missing an opportunity, but an extremely difficult one to spot when short of time. 33 Rcc2! Bxa3 34 Nf4 Qxb4 35 Rd7 launches a mating attack, with threats of Nxe6+ and Rcc7. One remarkable idea occurs in the variation 35…Qe4 36 Rcc7 Kg8 37 Rd8+ Bf8 38 Rcc8 Qb4 39 e4! and Black has almost run out of moves, e.g. 39…a3 40 Nd3 Qe7 41 Re8 Qd6 42 Rcd8 and wins. Bxa3 34 Rxb5 Qd7 A clever move, overlooked by Carlsen. 35 Rc5 e5 Ramping up the tension, but the simpler 35…Bxb4 was stronger, putting White on the defensive. 36 Rc2 Qd5 Again, 36…Bxb4 was stronger, though it is not easy to see a key idea 37 Rcc1 Ba3 38 Ra1 Qg4! indirectly defending the bishop. 37 Rdd2 Qb3 38 Ra2 e4 39 Nc5 Qxb4 40 Nxe4 40 Rdc2 ties Black in knots, with the idea 40…f5 41 Nxa4 Qxa4 42 Rc3. White wins the bishop, and the rooks will eventually hoover up the weak kingside pawns Qb3 41 Rac2 Bf8 After many more adventures, the game reached the position shown in the diagram above right.

Computer databases indicate that with perfect play, Black could draw with either 130…Qb1 or 130…Qc2. The idea is to prevent the advance of White’s king and knight up the kingside, e.g. 130…Qb1 131 Kg4 Qd1+ 132 Kh4 Qe1!, frustrating White’s ambitions. Instead, the game continued 130…Qe6 131 Kh4 Now Rf6 and Nh5 cannot be prevented. Qh6+ 132 Nh5 Qh7 133 e6! Qg6 134 Rf7! Kd8 134…Qxe6 135 Ng7+ Kxf7 136 Nxe6 Kxe6 137 Kg5 Kf7 138 Kf5! wins. 135 f5 Qg1 136 Ng7 The White king can shelter from the checks on g8, and the e-pawn decides the game, so Black resigns.

Once can only guess at Nepomniachtchi’s anguish after this painful defeat. Two days later he lost again, after putting up considerably less resistance. As I write this, Carlsen leads by 5-3 with six games remaining, so is an overwhelming favourite to retain his title.