The tennis star welcomed her daughter Alexis Olympia into the world with her husband Alexis Ohanian two years ago. Picture: Instagram

Serena Williams has praised her daughter as her “greatest accomplishment” as she rang in her second birthday.

The tennis star welcomed her daughter Alexis Olympia into the world with her husband Alexis Ohanian two years ago, and marked the special occasion with a touching tribute on social media.

She wrote alongside a picture taken on the day of Olympia’s birth: "The last 2 years have been my greatest accomplishment (sic)”

Serena and Alexis are Jehova’s Witnesses and so don’t celebrate birthdays, but the Reddit co-founder still took to Instagram to ring in Olympia’s second “cake day”.

He wrote: "How has it already been two years? Happy cake day @olympiaohanian. Thank you for being the greatest thing we've ever done. (sic)”

Last year, the 37-year-old sports star honoured Olympia’s birthday with a video that was taken while she was still pregnant, on the day she went to the hospital, where she was given an emergency C-section after her heart rate dropped dangerously low during her contractions. 

Just days previous, she had insisted she wouldn’t be celebrating Olympia’s first birthday for religious reasons.

She had said: "Olympia doesn't celebrate birthdays. We're Jehovah's Witnesses, so we don't do that.”

Williams has been a Jehovah Witness since her mother Oracene converted to the religion in the 80s but she's only just got more "into it".

She said last year: "Being a Jehovah's Witness is important to me, but I've never really practiced it and have been wanting to get into it. 

"Alexis didn't grow up going to any church, but he's really receptive and even takes the lead. He puts my needs first.”

Serena regularly attends church and is convinced she wouldn't have the success she does today in her sporting career if she didn't believe in God.

She explained: "I am Jehovah's Witness. If you don't believe in God - I think if you don't believe in God, it's going to be tough to live life because pretty much that's the basis of life, it comes from God. 

"And so being a Jehovah's Witness, obviously we believe in God and the Bible. And without Him, I wouldn't be here right now. I really thank Him for everything."