Serena Williams welcomes baby girl

Williams, who will turn 36 later this month, was admitted to the St Mary's Medical Center in West Palm Beach, Florida on Wednesday before going into labor overnight.

There was no immediate confirmation of the birth from Williams' representatives. 

Elder sister Venus Williams described herself as "super excited" after being informed of the reports concerning the baby as she walked on to the court for her third-round match at the US Open in New York.

However she declined to take questions on the subject in her post-match press conference.

"I'm definitely available to answer questions about tennis. That's all right now," Venus told reporters.

Earlier, a producer with the Florida-based WPBF television station cited hospital sources as saying Williams' daughter weighed in at 3.09 kilograms.

Williams, who is engaged to Reddit co-founder Alexis Ohanian, was swiftly greeted with an avalanche of congratulations from well-wishers.

Williams has vowed to return to competitive tennis in January to defend her Australian Open title, which she won while newly pregnant.