Serena Williams won the Australian Open while about 8 weeks pregnant. AP Photo/Aaron Favila, File. 

Tennis superstar Serena Williams has written a touching letter to her unborn baby, saying "I can't wait for you to join us in the players box next year".

Serena - the most successful female player of the Open era - revealed last week that she was twenty weeks pregnant - meaning she would have been in the first trimester of pregnancy when she won the Australian Open in January.

I can't wait to meet you

Now she has shared how much the news means to her. "My dearest baby" she begins the letter, captioning a photo showing off her baby bump 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." 

World's youngest number one

Referring to her return to the number one spot in the tennis rankings, she went on: "But most importantly, I am so happy to share being number one in the world with you.... once again today...from the world's oldest number one to the world's youngest number one. -Your Mommy"

Serena Williams announced in December that she was engaged to Alexis Ohanian, a co-founder of Reddit, the social messaging site. Following news of the pregnancy, her spokeswoman said the tennis superstar will not play again in 2017 but will return to the tour in 2018.