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

Just days after announcing that she's 20 weeks pregnant via a surprise Snapchat selfie, Serena Williams has taken to social media yet again—this time, to share a heartwarming note to her unborn baby that will have you weeping.

The tennis champion posted a new bikini selfie to Instagram on Monday, which shows her cradling her growing belly. The post is in honor of her fiancé Alexis Ohanian's birthday, but Williams's note is dedicated to her baby:

"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"

Of course, Williams is referring to the fact that she's literally one of the greatest athletes of all time. In September, she set a new world record for most Grand Slams out of any tennis player ever. At age 35, she's the oldest pro to hold the No. 1 spot, making her unborn baby the youngest. And Williams's baby had a front row seat for mom's latest win—Williams won the Australian Open while pregnant.

This sweet letter is only the latest exciting update from Williams. Since getting engaged to her Reddit cofounder beau in December, the superstar has shared sneak peeks of everything from her engagement ring to her wedding dress. We can only hope that she continues to serve up more info on all things wedding and baby as the weeks go on.

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.