At the elite level of sport, where most of the players are of similar ability, and have similar access to technology & training methods, it is the mental strength of the individual which separates the great players from the good players
The amount of mental energy or preparedness a person has prior to performance. Athletes performing at optimal arousal levels are said to be in "The Zone" / "Flow State"
Increased muscular tension can affect coordination; narrow internal focus results in missed environmental cues; inability to shift attentional focus to appropriate state