Serena Williams has announced she is 20 weeks pregnant, meaning she won the latest of her record-breaking haul of majors as a mother-to-be.

The US star, who is engaged to Reddit co-founder Alexis Ohanian, made the announcement on Snapchat on Wednesday.

Williams added the latest of her 37 major titles at the Australian Open, beating sister Venus to the singles title on January 28.

And while she won't have known it, the 35-year-old was expecting at the time.

Serena Williams announces her pregnancy (Snapchat)

Williams will go down as one of tennis' all-time greats and could well be the finest player to have ever played the game.