Serena Williams Chills with Bruno Mars Backstage, Mom's Night Out

Serena Williams, fiance Alexis Ohanian and Marc Anthony all rocked with Bruno Mars backstage before his concert in Miami Wednesday night ... but Serena's bod stole the show.

The tennis superstar just gave birth to baby Alexis a little over a month ago ... and she's already squeezin' into a skin-tight dress from gal pal Eva Longoria.

You could tell Serena and Alex were PUMPED to get some face time with Bruno backstage ... and later during the show, jammin' to "Just the Way You Are" with Marc.

But Serena says not even an "amazing" Bruno show can compare to mom life ... 'cause cuddlin' up with Alexis after the concert is "priceless."