Just make the right estimation of your own strengths and weaknesses, and also that of your opponent.
Anatoly KarpovPlaying black, I put great stake in the Ruy Lopez: I liked it, feel it, and understand it; in matches with Hjartarson and Timman it served me well. - on preparing for World Championship versus Garry Kasparov
Anatoly KarpovChess is a very tough game, and psychologically a tough game. And of course chess needs a lot of qualities, human qualities. And so you must have very strong nervous system and then you must be well prepared, you must be able to work a lot.
Anatoly Karpov