Serena Williams announces pregnancy just months after Australian Open win

World tennis champion Serena Williams has announced she is 20 weeks pregnant, meaning the superstar was already expecting when she won the Australian Open in January.

Fans of the 35-year-old were quick to praise the athlete on social media for her Australian Open win against her sister Venus, with many quick-thinking tweeters doing the pregnancy math.

The American athlete has not played since the Australian Open, when she would have been about 10 weeks pregnant, citing a knee injury as her reason to withdrawal from the BNP Paribas Open and the Miami Open.

Williams hinted of her pregnancy on Snapchat, posting a photo of herself appearing to show off her stomach with the caption "20 weeks".

While she was quick to remove the photo from her account, her publicist has since confirmed the news she is pregnant.  

"I'm happy to confirm Serena is expecting a baby this Fall," spokeswoman Kelly Bush Novak said.

She added that while Williams won't play again this year "she looks forward to returning in 2018".

In December she announced her engagement to Reddit co-founder Alexis Ohanian, 33, on the social media site Reddit.