When you
practice, you learn new skills and/or hone existing ones, going through the 4 stages of learning (from unconscious incompetence to conscious competence). The practice arena is also ideal for learning match playing skills, such as diff. strategies and tactics, mental aspects/training, aso.
The
competition arena is not a learning area, but a performance arena. This is where you play to win - and any thoughts regarding technique will easily have negative effect on your game.
However, the one aspect that is very hard to train, is the mental pressure present in match play. This, along technique-thoughts, are the two main components why many struggle with the practice to match transition. It is possible to train the pressure component, though: 25 pushups for a missed second serve, $10 penalty for each missed volley, aso.
This is clearly seen in golf, where many are able to produce great shots on the driving range and (training) putting green, but crumble under the self-imposed pressure on the course.
I read an amusing tip for training (golf) chip shots during winter time: set up in the living room, aim for the pillow in the armchair and put the (wife's) finest china in front. Pressure bonanza