But Osaka - who displayed not only a stellar game but remarkable poise throughout - held serve to seal a historic win for her country.
"She played well", Williams said, pausing to compose herself.
The multi-times grand slam victor wore a black bodysuit for the match specifically to assist with her health concerns after giving birth to daughter in 2017.
Speaking about her daughter in HBO documentary series Being Serena: "For so many years, I defined myself in just one way: by success, by championships, by making history". Not only did the ensemble make Williams feel like a "superhero" but it was also created to help her avoid blood clots-a medical condition she experienced after giving birth to her daughter.
It was in the second game of the second set that Williams was warned for receiving coaching, a charge she vigorously denied.
The tranquility didn't last long.
The video contrasts shots of Williams taking the court at Flushing Meadows this week with those of her in a hospital bed, cuddling her baby daughter.
"I didn't get coaching". Williams later broke her racket, which earned her another penalty, which led to an argument between Ramos and Williams. "I don't cheat to win". Williams joked her daughter also had winning on the brain by captioning the sweet photo, "Did Momma win?"
"If he gives me a thumbs up, he's telling me to come on", explained Williams.
During the post-match press conference, a tearful Williams said she will continue to fight for women's rights after claiming she had no regrets about her ugly fight with umpire Carlos Ramos over cheating claims in the US Open women's final.
It's not the first time emotional outbursts have cost Williams at the US Open. "No matter what happens in any match - semis finals, finals - I just feel like I've already won".