"The whole art of war consists of a well-reasoned and extremely circumspect defensive followed by rapid and audacious attack" - Napoleon.
This principle is equally true when it comes to war, martial arts or tennis. In martial arts, Bruce Lee stated that we should "be like water." Water is not rigid, it fills the space in which it is placed instantaneously. If a crack opens up in the container, the water rushes out through it. If the container remains intact, the water stays patient ans waits for the opportunity.
In tennis, it is important to develop strong defenses. A strong defense means that you are capable of being patient and waiting out the opponent until a crack in his armor appears. And, in the developing stages (and even some of the higher levels), it will not take long for the opponent to break down. However, when this happens, do not hesitate to capitalize on your good fortune (or goo strategy). Pounce on the opening immediately; do not remain rigid in your thought and movement; "go with the flow". When watching Federer, you can see how effortlessly he moves from defense to offense and vice-versa. He is neither rigid nor flustered when getting pushed around the court; it happens and he expects it. When the play calls for defense, he defends. When it calls for offense, he attacks. It seems that he never waits to execute 3 shots if 2 will do. Watching him move around the court, it seems as if there's a water bubble between him and the ball, he backs up slightly on deep balls, moves in to follow penetrating shots and side to side.