What is your commute/work life like? Assuming a 40 hour work week and either a 20 minute commute or one that you can bike to (for fitness sake), work doesn't seem like that much of a blocker.
Assuming like 10 hours a day for work + commute + getting ready in the morning, 2-3 hours a day for relationship/random stuff, and 8 hours a day sleep, that's still like 3 hours a day for tennis and leaves your entire weekend free to put a solid 6+ hours in each day.
If you don't like your job.. what about doubling up school/retraining with tennis, as it seems pretty complementary and you'd probably be happier AND might have more to show for it at the end. Worst case, future employers are a lot less likely to question a 6-12 month break "for school" than one where you just play tennis.
As a fellow DINK, I feel like I definitely could take off 6 months.. but it'd take me twice as much time working to break even due to expenses. So 6 months off would mean the next 12 months would be spent essentially paying for those 6 months w/ progress towards retirement/other stuff halted. YMMV depending on income/expenses/etc of course