A Fully Snappd Back Serena Williams ... Looking fit And Ready To Beat That Ass ..

You're looking at Maria Sharapova's worst nightmare -- a fully SNAPPED BACK Serena Williams ... looking fit, focused and ready to beat that ass ... less than a month after giving birth. 

Serena gave birth to little Alexis Jr. on Sept. 1 -- and spent several days in the hospital due to "complications."