US Serena Williams (L) leaves after attending her sister US Venus Williams' tennis match against Japan's Kurumi Nara at the Roland Garros 2017 French Open on May 31, 2017 in Paris. (AFP photo)

Paris: Serena Williams said Thursday that big sister Venus did not necessarily reveal that she is expecting a baby girl.

Venus, playing at the French Open in Paris, told a TV interviewer: "She's going to call me her favourite aunt."

"We all like 'baby V, baby Lyn, baby Isha'. We all want the baby to be named after us," the 36-year-old told Eurosport, referring to the names of Serena's sisters.

The interview led many to believe that the baby, due in September, will be a girl.

However, Serena took to Twitter on Thursday in an attempt to cool all those cooing on social media for a baby 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 you can ever have," wrote the 23-time major winner.

"So just to clarify what Venus said, I am always joking with my sisters to see what favours I can get them to 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' rather than 'he'."

Serena, who revealed she was pregnant in April, has always refused to discuss the sex of the baby she is expecting with fiance Alex Ohanian, the founder of Reddit.

She was at Roland Garros on Wednesday watching her older sister defeat Kurumi Nara in the second round on the showpiece Philippe Chatrier court.

Earlier this week in Paris, Venus had talked about her anticipation of becoming an aunt.

"I'm definitely looking forward to it. I hope that I can live up to this job," she said.

"I think you just have to be ready, aware, and alert. Then have a lot of diaper duty."

Serena, 35, learned she was pregnant just two days before the Australian Open.

She intends to resume her career in 2018.

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