In Team of Rivals, which is about Abraham Lincoln's presidency during the Civil War, there were so many instances where Lincoln's generals failed to pursue the retreating enemy after a decisive victory. History teaches us that the Civil War could have ended much sooner had some of these generals been more aggressive.
In the game of tennis, it's much the same. When you have an opponent on their heels, a big lead for instance, PUT THEM AWAY! Relaxing at the point of a big lead can give your opponent time to regroup and come back to beat you. Don't doubt that it can happen ... it can!
When you have the big lead, keep the pressure on. PUT THE ENEMY AWAY like a general and get off the court!