Serena Williams announced she is pregnant last week on Shapchat.
On Monday, she posted a photo of her growing baby bump along with a heartfelt letter to her unborn child.
"You gave me the strength I didn’t know I had," she wrote in the photo caption. "You taught me the true meaning of serenity and peace. I can't wait for you to join the player's box next year."
"But most importantly, I am so happy to share being number one in the world with you.... once again today," Williams wrote.
Williams is regarded as being one of the best players in the history of women's tennis and her talk of being "oldest number one" refers to the new WTA rankings that were released Monday. Those rankings put her back on top and bumped Angelique Kerber to No. 2.
In the post, Williams also gave a shoutout to her fiancé, Reddit co-founder Alexis Ohanian.