A 21 Move Brilliancy

The diagram shows the critical position from the game Fischer - Benko, U.S. Championship 1963. The game continued 15.Qg3 Kh8 16.Qg4 c6 17.Qh5 Qe8 18.Bxd4 exd4, when Fischer stunned Benko with 19.Rf6. Kasparov called the last move 'A spectacular zwischenzug' and awarded it '!!'. Benko resigned two moves later.

To beat a world class GM in 20 moves takes some luck and the notes of both Fischer and Kasparov indicate that the luck came on the 15th move. The move 15.Qg3 threatens 16.Bh6 Ne6 17.Bxg7 Nxg7 18.Qxe5, winning a Pawn. Black took the easy road out of the pin with 15...Kh8, but the main question is what would have happened after 15...f5.

U.S. Championship 1963
Benko, Pal

Fischer, Robert
After 14...Ne8-d6

Fischer wrote,

On 15...f5 16.Bh6 Qf6 17.Bxg7 Qxg7 18.Qxg7+ Kxg7 19.exf5 N6xf5 20.Rae1 Rae8 21.Ne4 with a comfortable edge, but no forced win.

White's 'comfortable edge' stems from Black's isolated e-Pawn and the nice square e4 in front of it, where White can park the minor pieces and make threats all over the board. Kasparov, repeating Fischer's analysis, wrote,

Benko fails to exploit a chance opportunity -- 15...f5. After this Fischer gave 16.Bh6 Qf6 17.Bxg7 Qxg7 18.Qxg7+ Kxg7 19.exf5 N6xf5 20.Rae1 Rae8 21.Ne4 'with a comfortable edge', but after 21...Nd6 it is still possible to defend. In addition, 19...N4xf5! is more accurate, with good chances of equalizing.

I don't see much difference between Fischer's 'no forced win' and Kasparov's 'still possible to defend', so it looks like the ex-World Champions agree. As for 19...N4xf5, I don't see much difference with 19...N6xf5, the square e4 still being weak. After 19...N4xf5, White also has the additional possibility of 20.Rf3. Kasparov mentioned that 15.Rad1 was stronger than Fischer's 15.Qg3, an assessment the computer agrees with. If that move had been played, there would have been no brilliancy.

Chessgames.com notes that Fischer beat Benko +9-3=7 over his entire career. The score was +3-2=5 going into 1962 Curacao, +2-1=1 in that candidates event, and +4-0=1 thereafter.