Positioning: is the foundation for almost all of your footwork when you are playing tennis, how you move around the court in one way or another relates back to the position. More generally, the position is an athletic position that allows you to move quickly around the court in any direction. The better you position yourself on the court, the more effective of your shots, aiming and balance will be better. If you run too close or too far to the ball, then your shots will most likely have bad timing or make errors.
Reflexes: If your reflex is quick, then these aspects of the game will be much better and effective:
Return of serve
How fast can you reach the ball (combine with speed and stamina too)
How fast or slow your nervous system act when you get or hit the ball
Shot-making (Drop Shot, Lob, Half Volley)
Reflex and Position are two of the most important aspects of the game. Nadal, Djokovic and Murray are 3 examples for having very quick Reflex and well Position of their games