Serena still in the dark about unborn baby's sex

'I think the surprise of knowing what you are going to have on that very special day you give birth is probably the best surprise possible.'

Serena Williams denied that she already knows the sex of her unborn baby, a day after big sister Venus appeared to hint that the former world number one is expecting a girl.

Serena, the winner of 23 Grand Slam singles titles, announced her pregnancy in April and is on maternity leave for the rest of the year.

The 35-year-old is engaged to Reddit co-founder Alexis Ohanian.

Venus referred to the baby as "she" and listed a series of girl names during an interview after her French Open second-round win over Japan's Kurumi Nara at Roland Garros in Paris on Wednesday.

"She is going to call me favourite aunt," she told Eurosport, while joking that her other siblings -- Lyndrea and Isha -- were involved in a light-hearted rivalry with her, with each sister trying to get Serena to name the baby after them.

"We're all like, baby V, baby Isha, baby Lyn... We all want the baby to be named after us," she added.

Serena, who is expected to give birth this fall, tweeted on Thursday that she is using that rivalry to trick her siblings into doing favours for her, but added that she is waiting for the birth to discover if her child is a boy or a girl.

"I think the surprise of knowing what you are going to have on that very special day you give birth is probably the best surprise possible," Serena tweeted.

"To clarify what Venus said, I am always joking with my sisters to see what favours I can get them do for me next. If I want something from one of my sisters, I say baby Venus, baby Lyn or baby Isha needs it.

"I did not grow up with any brothers, only sisters and we all say "she" more than "he"."

Image: Serena Williams

Photograph: Dia Dipasupil/Getty Images