Food is fuel.  The more energy one expends during an activity, the more fuel a person requires. In essence, sitting at a desk writing reports requires much less fuel [or food] than someone on the go running errands or breaking a sweat with an elevated heart rate for the same duration.  

Tennis matches may last one, two, three or more hours, so don’t enter the competition low on fuel reserves or empty. It is essential that athletes not only have enough energy to kick start the match, but also sustain enough energy to strike the final ball.

Kara Lydon, RD, LDN, Chobani Health Communications Manager, (the same yogurt company that John Isner is a fan of), says tennis players should eat one-to-three hours before a match to build up adequate energy. “Energy is best absorbed from foods that are higher in carbohydrates, moderate in protein and low in fat to promote quick digestion and absorption,” Lydon added.  However, take note that feasting just before court time clearly isn't a good idea; the less time one has before a match, the lower the volume of food that can be tolerated by our digestive system. It's also best to stick to tried and trusted foods – experiment with new snacks before a practice, not an important match.

 With that said, not all foods are equal and some snacks that might seem like a good idea actually won't promote optimal performance. Lydon offers a few tips on the best and worst snack foods on the market.

Tennis athletes should ideally fuel 1-3 hours before a match to build up energy for speed and endurance. 

Before a competition, tennis athletes should choose foods that are higher in carbohydrates, moderate in protein and low in fat for quick digestion and absorption. 

The less time you have before a match, the lower the volume of food that can be tolerated. Always experiment with new foods during practices to avoid digestive upset on the big day.

5 snacks to eat before tennis competition:

• Oatmeal with fruit. Ideal for a morning snack before an afternoon competition, oatmeal is well-tolerated and provides long-lasting energy. The added fruit will give you that quick jolt of energy you need to get your engine started. 
• Peanut butter and jelly or banana sandwich. Snacks, like small sandwiches, that contain a variety of carbohydrates help maximize and replenish glycogen stores. Adding peanut butter provides protein and healthy fat which can help to sustain energy throughout long-winded matches.
• Non-fat or low-fat Chobani Greek yogurt with fruit or cereal. Greek yogurt is an ideal real food pre-workout fuel that's packed with protein and carbohydrates. It's easy to eat, well tolerated and you can add fruit or cereal for an extra energy burst.
• Toast w/ slice of low-fat cheese. Low-fat dairy choices, like cheese, provide all three types of fuel – carbohydrates, protein and fat. Choose whole-wheat bread for long-lasting energy if you've got some time before the competition.
• Fruit smoothie with non-fat or low-fat Greek yogurt. Not only are fruits easily digested and provide quick energy but they're hydrating too. Smoothies are easy on the stomach, hydrating and serve as an easy vehicle for carbs and protein pre-competition.

5 snacks to avoid immediately before a tennis competition:

• Protein shake. Try to avoid protein powders and large amounts of protein before a competition to lower the risk of digestive upset. To be safe, save the protein shakes for post-competition when muscle recovery is key.
• Caffeinated drinks. Skip the sugary lattes before a match. Although some research supports moderate amounts of caffeine pre-exercise to enhance performance, caffeine can be dehydrating and hard on the stomach. 
• Whole-wheat pasta. Whole-wheat pasta can be a great pre-competition meal the night before or even 4 hours prior to the match when your body needs slow-releasing carbohydrates for long-lasting energy. However, immediately before a match, your body relies on quick energy from easily digestible carbohydrates.
• Nuts and seeds. Nuts and seeds are super healthy sources of fiber and fat. However, before a competition, it's important to focus primarily on simple carbohydrates and to limit amounts of fiber and fat to avoid any digestive discomfort during exercise.
• Salads. Leafy greens can be healthy complement to your balanced pre- or post-competition meal. However, it's best for athletes to avoid greens right before a match since they're high in fiber and not easily tolerated.