Serena Williams' daughter is about to get a lifelong doubles partner ... the tennis legend just revealed she and her hubby, Alexis Ohanian, are having another girl!!!
The couple made the announcement during a lavish party over the weekend ... that featured a prank cake, a dunk tank and an epic light show.
Williams and Ohanian documented the entire fiesta in a sweet YouTube video -- which showed Serena had been secretly hoping for a second daughter the entire time.