Williams, who confirmed through a spokesperson last week that she is expecting her first child with fiancé Alexis Ohanian, had not commented publicly on her pregnancy until posting a selfie with the touching message underneath.
"My Dearest Baby, You gave me the strength I didn’t know I had. You taught me the true meaning of serenity and peace. I can't wait to meet you," Williams captioned the selife.
"But most importantly, I am so happy to share being number one in the world with you....once again today."
The 23-time major champion, whose rep also confirmed that she intends to return in 2018, also added: "I can't wait for you to join the players box next year."
Ohanian, who had also been silent on the news until Monday, added his own sentiments about being an expectant father on Instagram after Williams' announcement.
He wrote: "My number one, @serenawilliams, is pregnant (she announced officially earlier today) and I'm so excited to meet the little one. You keep amazing me."