The pregnant athlete showed off her adorable burgeoning belly in a lemonade yellow swimsuit, which featured silver buttons on the side.
"20 weeks," the text over the photo, which she has since removed from her story, read. Williams' rep later confirmed the news to ET, sharing, "I'm happy to confirm Serena is expecting a baby this fall."
The official Twitter account for US Open Tennis also announced the news, writing, ".@serenawilliams will have a new pride & joy to hug and call her own soon! Congratulations on the exciting baby announcement! #USOpen"
The news comes less than a week after Williams, who is engaged to Reddit co-founder Alex Ohanian, gave fans a peek at what we thought was her sexy, summer-ready body. No sign of a baby bump here!