You’ve probably noticed that when your legs feel strong, you feel better. My doctor first talked to me about leg strength and the important of proper leg position while doing squats when I mentioned that I’m a triathlete. She told me to keep my knees directly over my feet when doing squats so that they don’t drift to the inside.
I began to wonder about [older] tennis players I know who have knee problems, and why professional tennis players, who play a lot and play hard, don’t seem to have as many knee problems. Answer: they have good trainers who know how to keep their knees healthy.
Torn anterior cruciate ligament (ACL) is serious; but apparently, it’s almost unheard of for a young player to suffer a torn ACL or dislocated kneecap from tennis according to Dr. Colvin, the chief medical officer of the United States Tennis Association.
Dr Clovin in the American College of Sports Medicine (ACSM) recommends that your knees should stay just as far apart as your feet – knees over toes, when standing and waiting for the ball or when moving.
To protect and strengthen your knees, hips, and legs, one of the best exercises is the squat; sometimes it’s called “The Mother of All Exercises” (besides the deadlift). Squats keep your knees healthy and strong, as well as your hips, ankles, feet, back, and lots of other muscles in your torso. When you have any time, at work, at home between tasks (or in the middle of a task), take a two-minute break and do a few squats.
It’s not necessary to use weights; bodyweight is fine.
Here are some pictures from popsugar.com that clearly show you basic body position. 
1) Stand with your feet just over shoulder width apart.
2) Keep your back in a neutral position, and keep your knees centered over your feet.
3) Slowly bend your knees, hips and ankles, lowering your legs to a 90-degree angle
4) Return to starting position — repeat 15-20 times, for 2-3 sets for beginners (do this two or three times a week)
5) Breathe in as you lower, breathe out as you return to starting position.
Make your hips strong enough to keep your knees in the right position during squats; the Clamshell exercise will help:
- Keep your core engaged! This will activate your abdominal muscles and protect your spine.
- You should only be rotating from the hips, not the lower back.
- Be sure your neck is in a neutral position so you don’t strain it.
- Rest your head on your lower arm, and use your top arm to steady your frame. Be sure that your hipbones are stacked on top of one another, as there is a tendency for the top hip to rock backward.
- Engage your abdominals by pulling your belly button in, as this will help to stabilize your spine and pelvis.
- Keeping your feet touching, raise your upper knee as high as you can without shifting your hips or pelvis. Don’t move your lower leg off the floor.
- Pause, and then return your upper leg to the starting position on the ground. Do 20 reps on each side.
Do this exercise when you are able, also, along with your squats. I think you will find it will improve the way you do your squat and will help stabilize your daily movements and help to preserve your knees.