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Keep yourself as hydrated as you can. Take advantage of sports drinks with vitamins and electrolytes to keep your energy up. This way, you’ll be able to perform as best as you can. You can also eat any other source of carbs. Also, consider a snack that has some protein. By eating carbs and protein, you’ll replenish some of the energy you’ve expended while playing. This will enable you to play longer and more effectively.  Get a quick fix of protein by snacking on a handful of mixed nuts. Dried fruit or a banana will give you a quick dose of carbs during a changeover. before your match. Set aside 5 or 10 minutes before your match to stretch. You can either do static stretching (stretching and holding your pose) or dynamic stretching (doing a variety of movements). If you already stretch, consider stretching a little more.  Do a chest stretch by holding your arms out as far as you can. Stretch your core by bending slowly to one side and then the other. Stretch your legs by reaching for your toes. The best exercises you can do are running or swimming. Cardio will help increase your endurance and make it so you can quickly move around the court. An aerobic workout will complement sustained cardio, increase your agility, and improve your reflexes. A good aerobic workout will include a variety of exercises. Ultimately, the more exercises and the more time you spend, the better.  Practice pogo jumps to improve your springing ability and to strengthen your ankles. Do some lunges to improve your lower body endurance and strength. Try jumping jacks to improve your core and and agility. Choose exercises that target your back, legs, core and shoulders. Lat pull downs, leg presses and resistance band moves are all excellent ways to focus on those areas. To work your arms, use weight machines with cables and pulleys that replicate arm movements in tennis.

Summary:
Drink water or a sports drink during changeovers. Eat a granola bar, energy bar, or nutritional bar every other changeover. Stretch Commit yourself to a cardio regimen for an hour 3 or 4 times a week. Practice aerobic exercises for half an hour 3 or 4 times a week. Do strength training a few times each week.