I have been fortunate to not have very threatening injuries, but the two times I was injured, they were both pretty nasty.
The first one was during my sophomore year in high school where I had a pinched nerve. It was horrendous. Sometimes my chest would cramp up for no apparent reason and I wouldn't be able to breathe. If I tried to breathe, it would feel like I had a knife hitting my lungs. At the same time, I could not lift my right arm up past my shoulders at all, making it completely impossible to serve. It took nearly 2 months of going to the chiropractor to finally fix it. I think what caused it was serving on a rather cold night without adequate stretching of my back and right shoulder. So as a warning to you all, make sure you stretch enough
The only other injury I had was a left ankle injury which I developed at the end of my senior year in high school. My shoes were pretty worn out and started to allow my feet to have too much room in my shoes. So after a few weeks of playing like this, I somehow injured my ankle. I had to lay off tennis/running in general for 2 months for it finally heal. I couldn't hit backhands at all