Excellent book with exercises and stretches targeted for tennis: Complete Conditioning for Tennis
, Roetert & Ellenbecker. (I count 11 stretches for the lower body and most involve the hip.)
Stretching reference with many insightful discussions, Stretching Scientifically
, - For example, 'if you can't squat it often indicates that your Soleus (one of the calf muscles) is too tight'.
Reference on muscles and their joint movements - Manual of Structural Kinesiology
, Thomson, get and older edition (14th or 15th) as they are just as good as references but much cheaper than latest edition that is used as a college text. Identifies each joint, the muscles that work the joint and the joint motions produced by the muscle shortening. Very clear illustrations.
Some tight and weak muscles that my many years of my tennis & the gym missed:
Stretch - often recommended. Done lying on back with legs in sort of a figure 4 pattern. Mine was/is too tight.
& other Quad muscles - The rectus femorus is one of the quad muscles, the only one that attaches above the hip at the pelvis. Too tight, it causes anterior pelvic tilt
. Most people do the common quad stretch (bend knee, grasp ankle and pull it toward butt) with a slight (flexed) angle between the upper leg to trunk.
With that slight angle the rectus femorus is missed by the stretch.
Read the first paragraph after "Common Sense Solution" for a description of this common quad stretching error that misses the rectus femorus.
Weak Gluteus Medius
, important for balance, ice skaters & dancers have well developed gluteus medius muscles. Special exercises with resistance bands, 'clamshell', 'monster walks', 'fire hydrants'.
You should become aware of the postural issues in the hip particularly those that might cause the leg to rotate or tilt in. These posture issues are important for long term health of the joints of the leg, particularly the knee. The hip is critical for leg posture. Life style issues, how you sit ,etc. may be very important for your posture. Also, muscles at any joint operate in opposing pairs of muscle groups for motion, agonists (the shortening muscles) and antagonists(more for control). Both the agonist and antagonist muscles should be balanced regarding length and strength. Search Eric Cressey for some discussions on posture.
I have posted several times in more detail on the above. Search- Chas Tennis Piriformis, Chas Tennis rectus femorus stretch, etc to locate for more detail and links. Search the internet using piriformis stretch, gluteus medius exercises, gluteus medius resistance bands, etc..
Much of the internet is just cut & paste from other websites and may contain errors so I recommend the above references which all were written by experts. A book on kinesiology is a great life-time reference for any athlete. Suggest that you learn the muscles, joints and motions and their proper terms, especially useful to speed up targeted internet searches. You can often find the books at the library or, if they are not in your local library system, request a copy inter library loan - ask at the information desk.