"I'm super excited," Venus Williams said when asked by ESPN's Pam Shriver about becoming an aunt again, moments before her third-round match. "Words can't describe."
Television station WPBF (West Palm Beach, Florida) reported that the baby was born at St. Mary's Medical Center in West Palm Beach.
"I'm so thrilled for her, what a role model she's going to be for that daughter," ESPN commentator Chris Evert said during Friday's afternoon broadcast of the US Open. "I told Serena this: It's a love you've never felt before, and I know she's probably experiencing that right now."
Venus, in her postmatch news conference after advancing into the fourth round at Flushing Meadows, requested that reporters stick to tennis.
"I'm definitely available to answer questions about tennis," she said. "That's all right now."
The questions turned to tennis and then family but eventually returned to Serena.
"We always talk," Venus said about her sister. "We always talk, so she's always encouraging me. Yeah, from every step of the way, every match, always."
Venus said she talked to Serena before the match, saying it was "more motivational" than advice about her opponent.
When asked whether Serena's daughter had a two-handed backhand and enough depth on her forehand, Venus cracked.
"Oh, my gosh," Venus said, smiling. "That's hilarious."
Serena revealed in April that she was pregnant, posting a selfie on Snapchat with the caption "20 weeks" and later confirming that she and fiancé Alexis Ohanian were expecting their first child.
She learned she was pregnant just days before the Australian Open began. For her 23rd Grand Slam singles title, Serena beat big sister Venus in the final.
In true Serena fashion, she has been adamant she'll return to the tour after having the baby.
Her coach, Patrick Mouratoglou, said in an interview recently that the plan, barring any complications, is for Williams to start training in November.
"The goal is to be ready for the Australian Open and then we'll see," Mouratoglou said of Williams' desire to defend her title. "If she's ready, she'll compete. If not, we'll delay the comeback. I don't think she should compete if she's not really ready."