My Favorite is a banana & a scoop of peanut butter about an hour before the match. It's not calorie dense, and the fat & protein in the peanut butter slows the absorption of the sugar from the banana (allows for a steady release of glucose into the bloodstream as opposed to a big sugar spike from just eating a banana)
Tennis is one of the most intense physical sports there is, at least once someone gets proficient enough to keep a ball in play for a respectable amount of time. I break from the notion that "carbs are the best source of energy", though. Foods that are very carb-centric (fruits, pastas, breads, cereals, etc) produce energy, but not sustainable energy for a long match. Walnuts, chicken, and other healthy fats and proteins produce as much energy, but typically last longer than fruit
or starchy carbs.
I'm not suggesting someone eat a 14-oz steak and then go play 3 sets, but with the right timing, eating a meal high in protein and fat a few hours ahead of a match is a great idea.
I generally eat this very dense whole wheat bread/cake whether I am playing tennis or not......very good when you are on the go.......but also now I have been thinking about also incorporating a high protein shake which includes a banana and all the liquid vitamins and minerals I am taking along with fish oil.....