In a match you cannot think about or experiment with your strokes. Practice is the time to experiment, but you should mostly rehearse your strokes so they become familiar and dependable. A rehearsed stroke is acquired through practice at a comfortable pace. An improvised stroke is a spontaneous reaction that occurs during match play or fast paced drills.
Do not rely on your ability to improvise. Rehearsing your strokes will never make you unimaginative. To the contrary, the more situations you rehearse, the more imaginative your range of options.
A tennis stroke is not a collection of disconnected actions. An efficient stroke is a series of actions that flow in an instantaneous sequence. Your muscles must be appropriately relaxed in order for the separate actions of the stroke to flow. Whether swinging or punching, the force of an efficient swing travels from your center of gravity to the racket head in an instantaneous sequence. A loss of balance at the beginning of the stroke will inhibit your body’s ability to initiate force. A loss of balance during the stroke will inhibit your body’s ability to transfer force to the racket.