Real Game Tactics 19: A Great Endgame Played By Fischer

This is a great endgame played by Robert James Fischer against William James Lombardy in the US championship 1960. In this tounament Fischer became US champion for fourth time.

The game: 

The puzzle:

If you don't see the whole game with commentary after solving the puzzle (so, when white doesn't do a move) , then click on the question mark below the diagram (if you click on it before solving, you will see the solution).

Black to move, how to win easy?