MIAMI, United States (AFP) — Pregnant tennis superstar Serena Williams was in labour at a Florida clinic on Friday, US media reported, prompting an outpouring of goodwill messages for the charismatic former world number one.

Williams, who will turn 36 later this month, was admitted Wednesday to St Mary’s Medical Center, which shut down an entire floor to provide security for the megastar, the local CBS affiliate said.

The network also said the 23-time Grand Slam singles champion — who revealed she was pregnant in April — was induced overnight.

“We’re sending all our love and best wishes to @serenawilliams, who has reportedly gone into labor,” the US Tennis Association wrote on Twitter.

The US Open — where Serena’s elder sister Venus is due to play Friday — also sent words of support.

“#HelloSeptember and soon to be hello to a new baby! Sending our best wishes to @serenawilliams who is reportedly in labor!,” the tournament wrote.

Williams and her fiance, Reddit co-founder Alexis Ohanian, have said they plan to keep the sex of the baby a surprise until birth.

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

Her spokespeople were not immediately available for comment on the reports.

– ‘Pretty intense’ –

Williams last month told Vogue in a wide-ranging interview that she had set herself a goal of returning to play in Melbourne at next year’s Open.

“It’s the most outrageous plan,” she said. “I just want to put that out there. That’s, like, three months after I give birth. I’m not walking anything back, but I’m just saying it’s pretty intense.”

Williams is one Grand Slam title short of matching Margaret Court’s all-time record of 24 Grand Slams and admits that she still dreamed of joining the Australian legend.

“Obviously, if I have a chance to go out there and catch up with Margaret, I am not going to pass that up,” Williams said. “If anything, this pregnancy has given me a new power.”

A separate article in Vanity Fair in June said Williams, who unveiled her pregnant body in a nude cover photo for the magazine, planned to tie the knot with Ohanian later this year after the birth of their child. The couple announced their engagement in December after meeting in 2015 in Rome.

In the same interview, Williams also admitted she was unprepared for the arrival of her child.

“I don’t know what to do with a baby,” she said. “I have nothing…. I’ve done absolutely nothing for the baby room,” she added.