Keep your knees bent

To maintain a flexible stance. One's knees should never be locked on any tennis shot, or even when one is not actually involved in a stroke. At the very least they should be slightly bent when waiting to receive, or standing at net. Bending the knees very considerably comes into play on any shot that has to be stroked near the ground. Such a shot cannot be made well from an erect stance. One must bend the knees almost to kneeling position and get one's body down to the ball.