The most difficult moment in a game is when the initiative changes sides. When you have been dictating the course of play, the sudden realisation that your advance has been halted can bring about a rapid deterioration of both morale and position.

Today's game from the current Madrid tournament is a good example. Yermolinsky, playing White, had dislodged Black's king in the opening and was looking for a way to breach the defences. If White's attack fails, then Black will stand better thanks to the fine square at d5 for his minor pieces.

In the diagram position, after Black's 22nd move, Yermolinsky played 23.Rd6. Bravo! After 23...Bxd6 24.exd6+ Kh7 25.Qe5 Rg8 26.Qf6 Black's position is a disaster. So San Segundo played 23...Qc7. White barged on with 24.Nd4, clearly thinking of sacrifices on e6, which was met by 24...Qe7 (24...Bxd6 25.exd6 is still disastrous for Black).

Now the moment of truth: White must have looked hard at 25.Nxe6+, 25.Bxe6 and 25.Qg4, but none quite works. But 25.g3? and 26.Nb5? was a disaster. After 26...Ba8! Black wins! At the end, 29.exd6 Qb7 is fatal - an echo of White's earlier ambitions to mate on the a1-h8 diagonal.

White: A Yermolinsky

Black: P San Segundo

1 d4 d5 15 0-0 h6

2 c4 e6 16 Rd1 Qb6

3 Nc3 c6 17 Bc4 Rc8

4 e3 Nf6 18 a3 g6

5 Nf3 Nbd7 19 dxc5 Bxc5

6 Bd3 dxc4 20 axb4 Nxb4

7 Bxc4 b5 21 b3 Kg7

8 Bd3 b4 22 Bb2 Rhd8

9 Ne4 Be7 23 Rd6 Qc7

10 Nxf6+ Nxf6 24 Nd4 Qe7

11 e4 Bb7 25 g3 Kh7

12 Qe2 c5 26 Nb5 Ba8

13 Bb5+ Kf8 27 Rxd8 Rxd8

14 e5 Nd5 28 Nd6 Rxd6 0-1