Reddit co-founder Alexis Ohanian turned 34 on Monday, the day that Williams also recaptured the women's top ranking.
Williams' social media post showed the 35-year-old sitting on a beach lounger in a black two-piece swimsuit that revealed her growing baby bump.
"My Dearest Baby, You gave me the strength I didn't know I had," Williams wrote on her Instagram account, as she hinted that she expects to be back playing next year.
"You taught me the true meaning of serenity and peace. I can't wait to meet you. I can't wait for you to join the players box next year. But most importantly, I am so happy to share being No 1 in the world with you.... once again today. On @alexisohanian bday... from the world's oldest No 1 to the world's youngest No 1. -Your Mommy."
