All of my players are trained to self-assess their ability to hit their targets at the right speed by observing their ball-watching skills in conjunction with having a FEEL for their muscle tension level. Not completely watching the ball is the #1 cause of errors in tennis ... and trying too hard is not far behind.
A part of our training for the upcoming season has been to practice more serves and returns. Nearly all of my students have responded extremely well to suggestions of relax and keep your head still at contact. Recently, one player served about 10-12 first serves in a row in the warmup only to begin our practice points with only 1/5 first serves in.
I asked her what happened and she responded: "They were going in during practice, so I thought it was working today."
What she neglected to realize was that the same mental process which grounded her practice serves MUST be brought along with her to the match! Each player may have a somewhat different mental process in the black box that's happening when they practice. However, it is crucial for executing to maintain this mental ritual during competition.
After a player has chosen what type of point they want to play, it is paramount to check in with "relax and see contact" just before the point starts. These two highly-related components of a strong mental game should then become your goal. Too often, in contrast, the intended target dominates the inner voice and execution falters as a result.
At the end of the singles practice which I just described both players were focusing on the right things and executing like champions. It was a pleasure to watch!
Relax and focus on seeing the contact ... the more that you practice it, the better your execution will become!