Williams, who was playing doubles with her sister, Venus Williams, also shared a video tweet zooming in on the baby, who was wearing a red and white headband.
Serena Williams made her long-awaited comeback to tennis on Sunday but the 23-time Grand Slam title victor and former world No. 1 suffered defeat, playing alongside big sister Venus in a Fed Cup dead rubber.
Every mom who's ever tried to ease back into exercise a few months after giving birth-especially after a hard C-section-knows how much of a challenge it can be to just pull on a pair of shorts, throw your hair in a messy bun, and take those first few tentative steps. All of this is fine, but more important than the details of the match is the fact that Serena's husband Alexis Ohanian and her baby daughter Alexis Olympia Ohanian Jr. were spotted in the stands cheering her on - marking baby Alexis' first public appearance! "That gave me an opportunity to be able to close it out for the US", she said. "I didn't expect to, I don't know, have that much power on my serves, even though they didn't go in". Both were in attendance over the weekend, seated just behind the players' box. I feel like I'm on the right track.
Serena told reporters on Friday that her path back had been full of "ups and downs" but credited her sister with helping her find her rhythm.
She had initially targeted last month's Australian Open and a defence of her 2017 title but struggled during an exhibition match against Jelena Ostapenko in Abu Dhabi in December, forcing her to revise her plans.
The 19-year-old Lapko made her Fed Cup debut in place of team No. 1 Aliaksandra Sasnovich, who lost her singles on Saturday.
But team captain Kathy Rinaldi confirmed her participation after Venus won her singles match against Richel Hogenkamp to seal overall victory for the US. "Or I'll get angry about the crying, then sad about being angry, and then guilty, like, 'Why do I feel so sad when I have a handsome baby?' The emotions are insane".
Still, Williams' attempted comeback will be a major story line in the sport this year, and fans will be watching closely.