The American, who is skipping the rest of the year after announcing last week that she was pregnant, took to social media to offer her first public thoughts on impending motherhood.
"My Dearest Baby, You gave me the strength I didn't know I had. You taught me the true meaning of serenity and peace," the 35-year-old who owns 23 grand slam titles, said on Instagram.
"I can't wait to meet you. I can't wait for you to join the players box next year," she continues.
"But most importantly, I am so happy to share being number one in the world with you... once again today. From the world's oldest number one to the world's youngest number one. - Your Mommy."
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Williams set off a day of frenzied speculation last Wednesday with a selfie posted on social media -- posing for a photo in a yellow one-piece swimsuit on Snapchat -- with the caption "20 weeks".
She deleted the post shortly afterwards but her Los Angeles-based publicist Kelly Bush Novak later confirmed that the player, who is engaged to Reddit co-founder Alexis Ohanian, was indeed expecting her first child.
It seems likely she was several weeks pregnant when she won this year's Australian Open to move to within one grand slam title of matching Australian Margaret Court's record of 24.
Her message on Monday appears to underline her resolve to return to action next year.
Despite not having played since January when she moved ahead of Steffi Graf as the most successful tennis player in the professional era by beating sister Venus in the Australian Open final, she replaced Germany's Angelique Kerber as world number one in the rankings on Monday.
(Reporting by Martyn Herman, editing by Pritha Sarkar)
