Serena Williams wrote a message to her unborn baby "from the world's oldest number one to the world's youngest number one" after returning to the top of the rankings on her fiance's birthday.

The legendary American has not played a competitive tournament since winning an Open Era-record 23rd grand slam title at the Australian Open in January, but moved back above Angelique Kerber on Monday.

Williams last week revealed that she is expecting her first child with Alexis Ohanian, so the 35-year-old's season is already over.

But she had another reason to celebrate on Ohanian's 34th birthday, just days after revealing she is due to give birth in September.

Williams posted on Instagram: "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.

"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"