The tennis superstar revealed the major news with a TikTok on Tuesday ... showing her get up from a table and leave the frame before returning with the little bundle of joy.
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Alexis and Olympia look happy as can be -- with big sis adorably giving baby Adira River a kiss on the forehead.
The Reddit co-founder also posted about the newcomer ... saying, "I'm grateful to report our house is teaming with love: a happy & healthy newborn girl and happy & healthy mama."
"Feeling grateful. @serenawilliams you've now given me another incomparable gift — you're the GMOAT. Thanks to all the amazing medical staff who took care of my wife & our daughter 🙏 I'll never forget the moment I introduced @olympiaohanian to her baby sister."
Serena and Alexis shared they were having another daughter at a lavish gender reveal just a few weeks ago ... and now that Adira is here, the countdown is on to see her take the court with Olympia -- just like Mama and AuntVenus!!