20 ... Ng6-f4 installs Black knight on the menacing f4-square Naiditsch vs Carlsen, 2006  (B33) Sicilian, 33 moves, 0-1 Fear of Nd4-f5 and kingside attack leads Black to offer a draw Carlsen vs E Alekseev, 2008  (E17) Queen's Indian, 18 moves, 1/2-1/2 2 games