Being 35 weeks pregnant ain't gonna stop Serena Williams from gettin' her sweat on ... 'cause the tennis superstar is still killiin' it in the gym during her 3rd trimester!!

Williams -- who announced she was 20 weeks pregnant back in mid-April -- hasn't slowed down one bit with a bun in the oven ... playing preggo tennis and posing naked for the cover of Vanity Fair.

Serena told TMZ Sports in May that she's not gonna retire once she gives birth -- and her mom says she's gunning to return to the court as soon as possible. 

G.O.A.T.

ALL GOOD IN THE FAM!
TMZSports.com