Serena Williams is enjoying an incredible time in her life.

The 35-year-old athlete is currently pregnant with her first child with fiance Alexis Ohanian, and on Monday, she was officially back in the top spot of her sport according to the Women's Tennis Rankings. Williams celebrated with a touching letter to her unborn baby.

"My Dearest Baby, You gave me the strength I didn’t know I had," Williams wrote in a sentimental Instagram post on Monday, alongside a selfie showing her growing baby bump. "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."

WATCH: Surprise! Serena Williams Is Pregnant!

And it looks like Williams and Ohanian, the co-founder of Reddit, are celebrating his birthday in style. The tennis star has been sharing pics of her enviable tropical location, and last week, shared an adorable photo of her fiance carrying her on the beach.

NEWS: Serena Williams Hits the Beach Following Pregnancy Announcement

Williams is taking her maternity leave from the sport after announcing her pregnancy last week. She plans to return to tennis in 2018.

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