Maria Sharapova -- HARDCORE SAND DUNE WORKOUT

Maria Sharapova is such a badass ... she hit the sand dunes BEFORE the ESPYs to sneak in a killer workout ... and we've got the pics.

The tennis superstar was out in Manhattan Beach where she ran up the sandy hills for about an hour ... showing off her killer abs in the process.

A short time later, Sharapova swapped out the gym clothes for a fancy dress and went to the award show -- where she towered over her co-presenter Floyd Mayweather in a pretty hilarious segment.