Rook versus knight is drawn when the knight stays next to its king in the centre. The rook cannot attack a protected knight with effect, and the king and knight protect each other.
A lone knight holds against a rook as long as it stays near its king and both stay away from the edge. Black's king on f6 and knight on e6 are the ideal pair: the king protects the knight, the knight covers the king's neighbourhood, and the pieces are in the middle of the board where the rook's checks have no sting.