Dartagnan: I would like to draw your attention on Serena's mucles during the tournament and Serena's muscles in normal life. 

On the picture below, you see, Maria Sharapova has finished a final and her muscles are normal. I mean they are not fully inflated.  Look the arms of Serena Williams at this final. Her muscles are fully inflated. 

But look Serena Williams' muscles in the picture with Nadal. Her arms are normal, they look like Maria Sharapova's arms at this final. 

Serena Williams might use a special drug to increase the power of her muscles during important tournaments. Serena Williams has the muscles of a male on this picture. It is not the case on the picture with Nadal.