In general, exercise is great for your brain. Experts have found that: “When you exercise, muscles begin to use oxygen at a higher rate, and the heart pumps more oxygenated blood through the carotid artery to the brain. In fact, the brain uses about 25 percent of the oxygen that you take in. Because exercise creates endorphins, people who exercise regularly have more energy, feel alert and have an increased sense of well-being and better memory retention.” (You can read this and way more details in this article: Mind-Body Exercise Connection.)
But tennis is not just plain, old exercise. It's a mental game too. In his post 34 Benefits of Tennis, Dr. Jack Groppel, Ph.D. and USPTA Master Professional, notes that, “Psychologically, the simple fact that you are practicing (or competing) one-on-one (or two-against-two in doubles) means that there is a human interaction necessary to perform. Studies have shown that one learns to manage adversity, improves problem-solving skills and learns how to manage mistakes and crises. Some studies have even shown brain growth through the problem-solving activities of tennis.”
So now you have another great reason to justify all that time on the court – you're just trying to get smarter!