Among many reasons (excuses if you wish) for tournament matches lost by all standards of players from novice to champion are carelessness and lack of experience in preparation....
The most important approach to a match is to be as normal as possible. True, the match is more important than an average game, and for that very reason most players do find themselves "on their toes" in spirit if not literally, but they should not approach a match very differently from a normal game. For instance, many players have lost tomorrow's match by retiring too early -- much earlier than customary without some unusual reason for tiredness. They have not been sleepy and while waiting sleep, have begun thinking of their match -- and remained in that state too much of the night.
How much should one sleep before a match? That is as individual as eating and drinking, and sometimes must be qualified by the time of play. Some tournaments begin early morning, and if a player is drawn for an early match, the time of the first morning meal -- and how much to eat -- has to be considered.
Some people digest food more easily than others. Keep well in mind that a match will not be played alone on the energy from food put into your body on that day, but fitness, strength and stamina built up over a period of time. If you are running late for a match, if you believe a morning meal is imperative before a morning match, then have a meal but cut down on quantity. Be careful: don't eat everything simply because it is placed on your plate.
You must get to know yourself in this respect: what you prefer to eat at certain hours before play and how much is best for you. Don't allow a stranger, trying to be kind, tell you what is best for you in this regard. it is important that you must learn and remember what is best.
If your match is delayed and you feel hungry, then have a little of something you like which you believe is good for you.
If the type of food you like before tennis is not available at the venue of the tournament, make sure you take along the next day whatever you wish.
If you are inclined to worry a little more than you think may be good for you and you believe watching tennis may tire you, take along something to read.
If the weather is cold take along the right kind of clothes in which to sit around and make sure you are warm when you step onto the court. Keep in touch with the tournament manager, asking for some short notice before you have to play so as to warm up by jogging a little, doing some calisthenics or hitting few balls if there is a practice court or a wall.
Your preparation should not finish there. If the weather is hot you may need a cap, salt tablets, sweatlets, something to drink, perhaps something special if you don't like water or soft drink supplied.
Don't be careless: don't take risks. Try to look at this preparation in the light of all your time, training and practice to reach the level of acceptance into the tournament.