The day before the tournament:
Player: "Can you feed me one more ball?"
Coach: "Sure."
One more ball turns into 50 more balls. With each extra shot, the stress starts to build, striving for perfection. With the tournament just around the corner, everything has to be perfect. Any innocnent bystander can recognize the player is starting to overthink things, needing things to fall in place exactly how they imagined. The player hits 4 fantastic shots, then misses by one by a foot, "one more, one more." The missed shots were no big deal a week ago, but today, those irky "feeling" shots start to irratate the player.
Everyone starts to feel a little insecure the day before the tournament. Doubts start to creep in and its only natural. Don't give it any value, just be aware of it. The rookie player will keep asking for "one more coach, one more." The veteran player pays no attention to any poor play because they understand today has no bearing on tomorrow. A great practice doesn't guarantee a great tournament. Often times, it can be negative if one has the best week of practice leading up to the tournament (it doesn't have to be negative, unless you let expectations build).
Sometimes the best tournaments are the ones where players have had a lot of adversity leading up to the event. Why? Low expectations and not needing things to be perfect. Tennis is a game about fighting, clawing, scratching to get your points- so just taking things as they come can really help a player relax and let their game come out.
The winning and losing in tennis is not controllable (if it were, you would win every match). This is why stress builds because one can never be certain of the outcome. If the match is a 50/50 match in your favor, the stress level is high. The player who needs things to be perfect is often times the player who loses- they overplay, berate themselves, and make poor decisions throughout the match. Emotions are flying out of control because they want to escape the stress. They want to get off the court (ie: tanking).
The next time you ask for "one more ball," please catch yourself. If you miss the last ball, call it a day. The sooner you can make peace with this type of reaction to a miss, the better off you will be.