Serena Williams waves to fans after winning against Simona Halep...

Serena Williams waves to fans after winning against Simona Halep of Romania in a quarter final match at the U.S. Open tennis tournament at Billie Jean King National Tennis Center on Wednesday, Sept. 7, 2016. Credit: Kathleen Malone-Van Dyke

Serena Williams says on social media that she gave birth to a baby girl named Alexis Olympia Ohanian Jr.

The tennis star posted about the birth on her Instagram and Twitter accounts.

She says the baby was born on Sept. 1 and weighed 6 pounds, 14 ounces. One posting also says about the newborn, “Grand Slam Titles: 1.”

The 35-year-old Williams won her 23rd Grand Slam singles title at the Australian Open in January — while she was pregnant.

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