Whatever else you do, read the following page from Sports Fitness Advisor The Elite Approach to Tennis Strength Training
http://www.sport-fitness-advisor.com/tennis-strength-training.html
Here are just a few highlights - read the rest for the big picture, plus plenty of detail.
"Unlike many sports, tennis demands several different types of strength... in particular muscular endurance and explosive power. And before these can be developed to optimal levels, the athlete needs to first develop good foundational and maximum strength.
If you try and train for every type of strength at once you'll end up with very little of anything - except fatigue!"
"Here are the 4 phases for this tennis strength training program example:
•Off-Season - 6-8 weeks
•Early Pre-Season - 6 weeks
•Late Pre-Season - 6 weeks
•In-Season - 3-4months
Each phase is covered in detail below.
The actual exercises, descriptions and routines for each phase are covered on a separate article..."
"Phase 1 - Foundational Tennis Strength Training
The objective of this 6 week phase is to build a solid base on which you build more intense, more tennis-specific fitness later.
Like all competitive sports, tennis places uneven demands on the body. You swing with one arm and one side of the body. Certain muscle groups are overworked while others are neglected.
Infamous over-use injuries like tennis elbow and damage to the rotator cuff muscles are less likely to occur in a balanced physique.
So our goal during this first phase is to prepare the ligaments, tendons and connective tissue for more strenuous activity to follow."
"Special Considerations in Tennis Strength Training
In sports like tennis and golf, overuse injuries of the wrist, elbow and rotator cuff muscles are all too common.
Most weight training exercises predominantly target the larger muscles groups. So while they get stronger and stronger, the smaller, more isolated muscles get neglected...
That doesn't normally cause a problem until you expose your body to thousands of repetitive movements that incorporate the larger AND the smaller muscle groups - like a forehand drive for example.
So while you hit harder and harder shots (as the strength in your large muscles groups increases), those finer muscles are placed under a disproportionate amount of stress.
The best way to compensate for this is to target and isolate those smaller muscle groups before they become over-worked.
By adding a few choice exercises for the forearm and rotator cuff muscles to your tennis strength training program, you can significantly reduce the occurrence of stress injuries in these areas.
You can start these exercises at any time or phase during the entire program. You can perform them at the end of a session or for 10-15 minutes on separate days."
So do the thrower's ten exercises now and for the rest of your tennis career
http://www.muhlenberg.edu/pdf/main/athletics/athletic_training/throwers10.pdf
What is nice about this site is that it keeps strength training in perspective, also emphasizing speed and agility training.
Still if you are really serious about strength training for tennis, the best specific regimens are in
Power Tennis Training [Paperback] by
Donald A. Chu.
And the best souce of overall information on tennis training is
Tennis Training: Enhancing On-court Performance [Paperback]
Mark Kovacs PhD (Author), W. Britt Chandler MS (Author), T. Jeff Chandler EdD (Author)