Tennis star Serena Williams has given birth to a baby girl, according to reports.

There has been no official confirmation of the birth, but asked how it felt to become an aunt again ahead of her third-round US Open match, Serena's sister Venus told ESPN: "Obviously I'm super excited. Words can't describe."

Serena's coach Patrick Mouratoglou wrote on Twitter: "Congratulations Serena Williams for your baby girl. I am so happy for you and I feel your emotion. Recover well & enjoy without limitation."

The US Open also tweeted a video message of congratulations to the tennis star at the news.

It comes a day after reports Serena, 35, had gone into labour at St Mary's Medical Centre in West Palm Beach in Florida.

Serena's fiance, Alexis Ohanian, the founder of Reddit, said in August he believed the 23-time grand slam champion would make an "awesome" mum.

Messages of congratulations soon flooded in following news of the birth.

Stacey Allaster, chief executive of the US Tennis Association said: "Hey Serena, welcome to the best club in the world: motherhood. Can't wait to see you, a mum, and your precious little healthy baby. Congrats!"

Spain's tennis superstar, Rafael Nadal also tweeted: "Congrats Serena!!! So much joy for you!!!"

Last month Serena, whose coach says the star plans to compete in the Australian Open in January, hosted a 50s-themed baby shower.

Among the stars who attended the party were Eva Longoria, Ciara, Destiny's Child star Kelly Rowland and La La Anthony.