The 23-time Grand Slam champion, along with sister Venus Williams, was raised as a Jehovah's Witness, a subsect of Christianity. An important aspect of that belief is that those who practice the religion do not celebrate birthdays.
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Williams' sisters converted in the 1980's, have followed the religion since
Both Serena and Venus followed their mother, Oracene Price, in converting to the religion in the early 1980's. On multiple occasions, Williams has spoken about her religious beliefs.
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"Being a Jehovah’s Witness is important to me, but I’ve never really practiced it and have been wanting to get into it," said the former World No.1 in 2017.
Following her marriage and birth of 1st child Olympia, Williams re-affirmed her decision to not celebrate her daughter's first birthday in 2018.
"We're Jehovah's Witnesses, so we don't do that," she said at a press conference.
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Furthermore, she stated her husband Alexis Ohanian is very supportive of her decision.
"Alexis didn't grow up going to any church, but he's really receptive and even takes the lead," she added. "He puts my needs first."