Serena Williams. Picture: Kin Cheung/AP

Serena Williams insists she doesn't know whether she's having a baby boy or girl.

The 35-year-old tennis player is expecting her first child with fiance Alexis Ohanian, and though her older sister Venus Williams appeared to let slip they are expecting a baby girl during an interview on Wednesday, May 31, when she joked about wanting the baby to be named after her, the sporting champion admitted her sibling was simply expanding on a family joke.

Serena posted on Instagram: "I think the surprise of knowing what you are going to have on that very special day you give birth is prob the best surprise you can ever have!

"So to clarify what Venus said, I am always joking with my sisters to see what favors 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 brothers, only sisters and we all say 'she' more than 'he'. Unless I'm joking with my dad or Alexis and I'm trying to con something out of them too. Hey I'm the youngest of five. I've always tricked all of them, what can I say! (sic)"

Venus sparked the speculation her sister is having a girl when she referred to the baby as she and said: "I want to be known as the favourite aunt.

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

Serena revealed her pregnancy earlier this year when she took to Snapchat to post a photo of herself with the caption "20 weeks", but recently revealed the photo was a mistake and she didn't mean to make it public.

She said: "Well, actually it was an accident. I was on vacation just taking some time for myself and I have this thing where I've been checking my status and taking pictures every week to see how far along I'm going.

"I didn't tell a lot of people to be quite honest and I'd been saving it. And you know how social media is when you press the wrong button ... and [there it was]."

Bang Showbiz