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One of Serena Williams most A-list pals Beyonce Knowles took to Instagram Friday to congratulate her friend on the birth of her baby girl.

Bey posted a shot from a Vogue photo shoot and wrote on the photo "Congratulations, Serena!"

Of course Beyonce just became a second- and third-time mom herself, with the birth of twins Rumi and Sir back in June. And Bey and the Queen of Tennis have a fairly long friendship history -- Serena even took to the stage during Beyonce's October Formation tour date in New Jersey back in October and Williams appears in Bey's "Sorry" music video.

Of course Serena's sis Venus is also issuing her congratulations, but, as one of the leading ladies at the US Open, she's made it clear to sports journalists who info on her sister that she's there to play tennis and to talk tennis and not family life. But she did cave yesterday and said, "I'm super excited ... words can't describe." Well okay, that's something!

Meanwhile Serena and fiance Alex Ohanian have yet to release the name they've chosen for their baby girl ...