The standoff was not one on one, of course, as tennis rules prohibit direct competition between men and women, but a competition of mixed doubles, in which Serena Williams played alongside Frances Tiafoe, while Federer played with Belinda Bencic.
The match will be the first time the two 37-year-olds, who have with 43 Grand Slam singles titles between them, face each other.
Williams fired a compliment right back at the male champion, calling him the "the greatest of all-time to be honest" and that is "both on the court and off the court".
" I was nervous returning because people talk about her serve so much".
"I was so excited, and literally it was the match of my career", Williams said of her joy of playing Federer.
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I am the one who has won those doubles grand slams but it looked like he was the one who won all those doubles grand slams today, but it was what it was. Federer captioned his "Oh what a night", while Williams wrote: "The GOAT vs the GOAT". For me it was super cool!
"She's a great champion, you see how focused she is and I love that about her".
The two Tennis greats even had time for a selfie.
"I can't read yours either", Williams joked.
Later with the set at 2-2, Williams fell to her knees and screamed in dismay as she missed an easy volley next to the net to drop the point. "You just can't read it".
"I think that's what it was made for, so that the greatest players in the world play each other. I wanted to take pictures and bring my baby out", she said.
Last year American tennis legend John McEnroe scoffed that if Serena Williams' record were to be measured by the standards of male players, she would rank a very low 700th in the men's circuit.
The winners of each group will contest Saturday's final.