Beyoncé congratulated Serena Williams in an Instagram post. Picture: AP

Serena Williams has given birth to a baby girl.

The tennis star and her beau fiancé Alexis Ohanian have reportedly welcomed a little girl into the world, weighing 6 pounds, 13 ounces.

A source from the St. Mary's Medical Center in West Palm Beach are said to have confirmed the arrival to WPBF-25 News.

US singer Beyonce, who gave birth to twins Sir and Rumi on 13 June, took to social media to congratulate first time mom, Serena.

A post shared by Beyoncé (@beyonce) on

Serena previously revealed she is "looking forward" to motherhood and thinks having a baby will make her a "real woman".

She shared: "I have so much respect for so many women [for giving birth]. I am about to be a real woman now, you know? It's going to be something incredibly impressive to go through ... I don't think watching birthing videos helps. I actually think it makes it worse. Having a baby, nothing is guaranteed."