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Read Serena Williams' touching letter to her baby: 'I can't wait to meet you'

Tennis superstar Serena Williams is already expressing tons of love for her baby.
/ Source: TODAY

Tennis superstar Serena Williams is already expressing tons of love for her baby.

The expectant mother shared a touching note to her unborn child Monday on Instagram.

“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,” she wrote in the caption of a photo showing off her growing baby bump.

Williams, the world’s top-ranked female tennis player, revealed last week in a Snapchat photo that she and her fiancé, Reddit founder Alexis Ohanian, are expecting their first child. Williams, 35, is due some time this fall.

MORE: Serena Williams is pregnant, shares baby bump pic at '20 weeks'

“I can't wait for you to join the players box next year,” she said in her Instagram post. “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.”

Dad-to-be Ohanian also shared his excitement in a separate post he put up on his social media accounts, including Reddit and Twitter.

"A new Snoo is on the way!" he wrote, referring to Reddit's mascot, in a caption of a cartoon drawing of him with his arm around a visibly pregnant Williams.

RELATED: Serena Williams shares new photo from her babymoon vacation in Mexico

The Women's Tennis Association on Monday ranked Williams again at No. 1. In January, she claimed the Australian Open title after defeating her sister, Venus Williams, in the finals.