Serena Williams dusted off the poodle skirt and saddle shoes to travel back to the Fabulous '50s for her baby shower over the weekend ... but not without the help of her superstar friends!!
Williams is expecting her first child with Reddit co-founder Alexis Ohanian -- who also played along with the style ... looking something like Danny from "Grease" with his leather jacket.
Serena announced she was 20 weeks pregnant back in April ... so the baby G.O.A.T. should be on the way shortly.