Tennis is like a game of chess. Your opponent makes one move and you react and make your move. It is an ongoing battle of action and reaction. The ability to read your opponent and the situation is crucial to success. Those players that have great "IQ", "Smarts", "Court Awareness" are excellent at reading the situation and making good decisions.
Decision making, like many things in sport, is affected by a player's focus, stress, confidence, goals, game style to name a few things as well as by the opponent, momentum, score and so on. If a player can impose his or her game plan on an opponent chances are that he or she will win. However, if the opponent can dictate play chances are that you will lose.