Three legged retirement
If you aren't near retirement now then you can question how much social security you may receive. If you have a 401K or IRA your investments may not work out as planned. Almost no one gets a pension unless you work for the government in some way. For the rest of us, and especially tennis teachers who are self employed your only way to prepare for retirement is to save well, invest, and consider some form of private pension by allocating some of your retirement funding in an annuity.
Not saving enough is biggest financial regret
Creating your own three legged retirement means considering each as a part of the whole, but adding a fourth leg by working in retirement can become the way to safely navigate your reclining years. As a tennis teacher your ability to just slow down and take less work while still working and generating income is a sound approach to making it through.
In addition, just having something to do that keeps you involved is a great justification for teaching long after many others might have quit their 9-5 jobs.
This is how I approach retirement. I just shared my ideas of how I have worked to navigate the sometimes difficult idea of having a comfortable and successful and winning fifth set in the grand slam event of life. Winning in this case means you live life as you choose.
Disclaimer: I am not an investment or retirement professional so please do not consider this investment advice but rather ideas for discussion. Seek professional assistance for your retirement planning.