As stated by lot of people on this forum there is no answer to this question or I should say there is no right answer?
To give a quick background about myself, I come from an athletics background and have been involved in athletics for the past 30 years. As an athlete I competed at an elite level and I have been a coach for the past 15 years. I coach sprinters and also work with tennis players to improve their speed and agility.
Being athletic does not guarantee success in the tennis court but it for sure helps you to learn the skills quickly and potentially progress faster. Being athletic usually means that the kid is better co-ordinated and adapts to changing situations on the court faster. Also the co-ordination and quickness (at least the basics) are best learnt between 8 to 12 years of age. So question is how much time you want to focus on making the kid more athletic.
The main issue is the lack of understanding on how to learn an athletic skill and knowing what short comings does the player has to learn that skill. For example I would hear the tennis coach yell at the player ‘Be explosive off the first step” or “use your legs to hit the shot”. Looking at the kid you can see that there is no way the kid can do all that because he is flat-footed and has no co-ordination. So does not matter how much work this kid will put on the tennis court those aspects are not going to be fixed. It has to be done outside off the tennis court. So for a player like that you are better off distributing your time equally between the tennis sessions and off-court practice. And contrary to people’s belief you can improve a kid’s athletic ability. You will not be able to make him a Usain Bolt or jump like Michael Jordan but can definitely make massive improvements in their atheism.
So in summary continuing to do an activity repeatedly does not guarantee that the player will improve. But understanding what is required to perform the activity and improving those pre-requisites might give you better results.