'My heart dropped': Serena reveals pregnancy shock

Serena Williams is the most decorated tennis player in history with 23 singles Grand Slam titles and 14 doubles Slams.

She dominated her last Slam, the Australian Open, with a huge secret — she was pregnant.

In a detailed interview with Vanity Fair, for which she posed naked for the cover, Williams has revealed how she felt different in practice sessions leading up to the tournament.

She was getting tired when she shouldn't, her body underwent some changes, and she threw up.

A friend of hers suggested she take a pregnancy test, to which Williams immediately dismissed the idea of being pregnant.

"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?" Williams said.

NOT BACKING DOWN: McEnroe throws fuel on Serena fire

She took the test and promptly forgot about it, leaving it in the bathroom for an hour and a half. When she remembered and came back to check it, she was shocked.

"(I) did a double take and my heart dropped. Like literally it dropped," she said.

Williams said she recalled thinking, "'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."

Williams said she immediately called fiancé Alexis Ohanian and told him to come to Melbourne. Thinking Williams had a scary medical diagnosis, the Reddit co-founder flew out to meet Williams.

He was stunned but estatic when she handed him a paper bag filled with six positive pregnancy tests.

At that point, it was deemed safe for Williams to play in the tournament. Only five people knew she was pregnant, including the doctor and Ohanian. Her coach nor the tennis officials had any idea.

But in pure Serena Williams style, she won all seven of her matches in straight sets to earn her seventh Australian Open and 23rd singles Slam title.

Now she is preparing to not only become a mother, but to marry Ohanian after the baby is born at the end of the summer. However, she isn't done with tennis yet.

"I don't think my story is over yet," Williams said, along with her plans to return to the tennis circuit by January, just a couple of months after the birth of her child.