Serena Williams poses naked and pregnant on cover of Vanity Fair

Serena Williams and her partner Alexis Ohanian. Photo / Getty Images

Serena Williams has admitted her "heart dropped" when she learned she was pregnant, initially thinking: "'Oh my God, this can't be...I had planned on winning Wimbledon this year".

Williams, who is pregnant with her first child with her fiance Alexis Ohanian, said she had only taken a pregnancy test on the insistence of a friend, and did not truly believe the result until she had done six.

In an interview with Vanity Fair US, she said she had delivered the news to Ohanian after asking him to fly from San Francisco to Melbourne, where she was about to play the Australian Open, and handing him a bag with the tests in.

Williams has now posed for the magazine's cover in celebration of her pregnancy, covering her breasts with her hands in a near-nude shoot by photographer Annie Leibovitz.

She said she had first noticed feeling physically different a week before the Australian Open, when her friend, Jessica Steindorff, bought her a pregnancy kit.

Serena Williams on the cover of Vanity Fair. Photo / Annie Leibovitz for Vanity Fair

Williams said she replied: "I'll take it just because (a) to prove you wrong and (b) because it's fun, whatever. It's like a joke. Why not?"

After seeing the results, she "did a double take and my heart dropped. Like literally it dropped.

"[She thought]'Oh my God, this can't be-I've got to play a tournament. How am I going to play the Australian Open?' I had planned on winning Wimbledon this year."

Steindorff procured five more tests, which all proved positive.

Now six months pregnant, Williams said: "It just doesn't seem real. I don't know why. Am I having a baby?

Serena Williams and her partner Alexis Ohanian. Photo / Getty Images

"I don't know what to do with a baby. I have nothing ... I've done absolutely nothing for the baby room."

She joked: "I'm trying to enjoy the little freedom I have left."

Ohanian, however, is "earnestly preparing and already has a tip jar he puts money into whenever he uses profanity so he won't utter it around the baby", the magazine claims.