And below, for reference, is Naomi's Circle. Naomi's goal is to hit balls that land outside this circle and inside the court. My error rate shoots through the roof when I try this.
(More like Naomi's Elipse, roughly (x^2/110.25 ft) + (y^2/225 ft) = 1)
Perhaps such hitting outside this circle is necessary, at times, to succeed at the highest levels of tennis. My collection of ATP Tour players hit far more inside the circle than outside it, but perhaps they are able to hit outside the circle when the opportunity arises. They do hit a lot of shots outside Naomi's Circle.
What does all this mean for a player? I'm not sure. I think aiming neutral balls outside Naomi's Circle is suicide. Aiming outside Naomi's Circle on easier incoming balls where your goal is to seize control of the point makes more sense – if you are tremendously skilled. I think for the vast majority of tennis players, ever aiming outside Naomi's Circle is a bad idea. Getting an easy ball and taking an enormous risk looks like a recipe for failure to me.
Instead, my philosophy is to aim for the center of the ATP Tour player's shot location scatter diagram. There are two distinct areas they aim for: roughly five feet inside the sideline and six to nine feet inside the baseline on each side. They do not appear to target in the middle very often.
By aiming for these relatively safe targets, players will reduce their own errors. This alone is enough for most players to win most matches. By win, I mean allow the opponent to lose.
For players who need to hit winning shots to win points and matches (less than 20% of all players at a minimum), the conservative targets still make sense. As their shots miss their targets and scatter, roughly half of the misses will be closer to the sidelines and the baseline. Those happy accidents present a chance to take control of the point with more forceful shots, but still aimed for conservative targets. Perhaps those happy accidents will produce balls short enough to attack and finish at the net. Perhaps they will elicit looping replies that can be taken out of the air with overheads or drive volleys from the mid-court.
What about the half of the errors that stray further from the sidelines and the baseline? Well, for 80% or more of players, those are just fine. They are still in play. Sure they may present the opponent an opportunity to take control of the point or attack. But most players lack the skill to do that, so such an opening does them more harm than good! For the highly skilled players, some of these errant shots into the middle of the court will put them on the defensive. That's why we practice defending with looping balls, slices, passing shots, and lobs. Defense is part of tennis, too.
If you aim near the lines, you had better be very highly skilled or comfortable with losing. And from the looks of those charts, the best players in the world only occasionally hit, much less target, near the sidelines. So "very highly skilled" may mean "more skilled than Nadal, Djokovic, and Federer." If that's you, thanks for reading this, and start composing that Hall of Fame induction speech!