It is helpful to consider an equation sometimes used by sports psychologists:
In chess terms, the meaning of this is that a player's performance from one tournament to the next will vary entirely with his motivation. Knowledge builds up slowly over the years and does not vary much in a short time, whereas motivation has massive swings up and down, accounting for good and bad form. Of course, other factors, like luck, can play a part and the above equation only holds as a rough approximation. Looking at my own results in recent years I have averaged about 2475 with several results over 2600 and several below 2350. In a match between myself in good form and myself in bad form, the statistical implication of the above figures is that good form would win something like 8½ to 1½. The moral is clear; competitive players should treat their motivation very seriously indeed and look for ways to improve it. It is not only intelligence, talent for the game, knowledge and understanding or physical fitness that count; will to win, pure unadulterated motivation can also count for enormous variation in the levels that different players reach.
Performance = Knowledge + Motivation