Listen to SA!
First warm up the body with movement.
Then if there still is an area like your shoulder that still feels stiff, warm that area up with movement - moving/swinging your arm first without your racquet, and then with your racquet.
Here's two articles from the USTA Sports Science section:
Strength & Conditioning: Dynamic Warm-up and Flexibility Training: //www.usta.com/Improve-Your-Game/Sport-Science/114698_Strength__Conditioning_Dynamic_Warmup_and_F lexibility_Training/
Dynamic Warm-Up http://www.usta.com/Improve-Your-Gam...ynamic_WarmUp/