This is all good advice, thank you. I saw an ortho doctor, he recommended weight training, which I am doing, and cycling is my favorite cardio, so I need to do more of that.
Basically, I guess "exercise the thing up to the point where it starts hurting" is the general advice for this kind of thing. I just hope the pain can go away so I can enjoy tennis again.
You can’t foam roll or stretch away IT band issues, fascia just doesn’t work that way.
In the past I had some issues with my IT band when I used to run longer distances. I was able to clear it up by using a foam roller in combination with other stretches. These days I have issues with my achilles/calf, which I’m sure I could clear up with a combination of strengthening, stretching, and weight loss - but instead have just been sticking to slower speeds and shorter distances. Cushioned shoes helped, too.
It may be worth seeing a sports or physical therapist. It’s amazing how much of a difference a strengthening exercise can make. Or if you’re like me and don’t like seeing doctors, there are a lot of physical therapy videos on YouTube to try out.
Last suggestion I have is to look into cycling. I ride with lots of guys who took up cycling after ACL injuries, achilles injuries, etc., forced them to give up other sports. With a proper bike fit, I think you shouldn’t have any issues with your knees. And can be done both indoors and outdoors (smart trainers and Zwift have come a long way in making stationary bikes more fun, though still not as fun as outdoors).