If your sports skill is well learned, your execution is automatic. But, it can be “de-automatized” with fatigue. To prevent this from happening you should be sure that you are in good physical condition so that you can prolong the onset of fatigue as long as possible. Doing so will allow you to execute your skills well and remain performing well.
However, contrary to popular opinion, it is not always the cardio-vascular system that gives out first: it is the respiratory system! Once your breathing is inadequate to perform as needed, cardiovascular system fatigue begins.
Because of this, to prevent early respiratory fatigue, you should train with a device such as the Sports Breather in order to strengthen the respiratory muscles. This is the key to preventing the onset of fatigue and improving your oxygen uptake.
In addition, with fatigue the central nervous system is disrupted and is on a low energy level. As a result you should never train technique when you are in a fatigue state. Although many coaches believe that they can sharpen shooting or hitting skills when a player is fatigued, this is non-productive. You should always practice when you are fresh and energetic, if you wish to improve your skills.
Some skill training in partial fatigue may be of benefit to better players who have well-established technique. However, for best results get yourself in good physical condition to prevent or delay the onset of fatigue and to display your best skill execution possible.
For more information on breathing in sports see Build a Better Athlete and Biomechanics and Kinesiology of Exercise.