Now that I'm beyond my warrior years, I have a keen appreciation for the things that keep me stronger and more functional on the courts. Even when I was younger, I would maybe ride my bicycle to get around, but now it's my #1 leg workout. If I get a moderate ride at least twice a week, my endurance is much better overall and my legs are typically issue-free.
I ride a road bike and have shoes with those "cleats" that snap onto my pedals, so my legs can both push down and pull up to turn them gears and work more of my legs. Cycling is about as low-impact as we can get and I've seen more than just one or two pros (not just tennis players) mention it as part of their conditioning package. Many athletes love how the bike doesn't put any additional pounding into their legs... works for me.