US tennis great Serena Williams on Monday marked her fiance's birthday and her return to the world number one ranking by posting an emotional message to her unborn baby on Instagram.

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 number one in the world with you.... once again today. On @alexisohanian bday... from the world's oldest number one to the world's youngest number one. -Your Mommy."

Williams is due to give birth in September, the month she turns 36 years old.

In December, Williams announced her engagement to tech entrepreneur Ohanian and last week revealed she is expecting her first child and will not play again this year.

She was already expecting when she won her record-setting 23rd Grand Slam title at the Australian Open in January -- where she beat her elder sister Venus in the final.

Kelly Bush Novak, Williams' publicist, had said this week that the US star "looks forward to returning in 2018" and that was echoed by her coach Patrick Mouratoglou.

Overtaking Australia's Margaret Court for the all-time record of 24 Grand Slam singles crowns -- Williams is on 23 -- is also "a determining factor" in her motivation to plough on, the Frenchman said.