And social media lit up with celebrities and athletes from the tennis world eager to offer their congratulations to the proud parents.
One of the first stars was mother-of-three Beyonce, who shared a snap of the 35-year-old during her pregnancy wearing a white dress with a billowing train. The words 'Congratulations Serena!' were superimposed on the image.
'You Reddit here first!' Wendy Williams joked referring to Alexis. 'Congratulations!
Men's champ Rafael Nadal tweeted: 'Congrats Serena!!! So Much joy for you!!!'
Serena's coach and TV commentator Patrick Mouratoglou posted three tweets, saying he has three daughters of his own and 'each time it's an incredible journey.'
And he added some advice, saying: 'I wish you a speedy recovery - we have a lot of work ahead of us.'