Tennis, like most sports, is played from the ground up. Transmitting force, gracefully, and in balance is the key to accurate shot making results. In mechanical terms, “gracefully” means without discontinuities or what a coach would call “hitches” in the motion. Minimizing the motion’s unnecessary stops/starts is the major factor to its efficiency.
Balance gives the billions of eye/hand calculations the best chance of accuracy. That is why if an athlete is off balance, their performance must take on more conservative/defensive choices in shot selection.
The more often the player is connected solidly to the ground during ball contact, the more often that player can dictate play and remain on offense. Lack of balance usually, but not always, means the player is on defense.
Interestingly in this sport, hand skills, dexterity, and experience responding to the opponents position, can make up for a deficit in strength, mobility, and foot speed. Age can often triumph over youth for this reason. On and off the court, knowledge is power!