Federer and playing partner Belinda Bencic overcame Williams and Frances Tiafoe in the Fast4 format and Switzerland beat the USA 2-1 in the annual team tennis event, in front of a capacity crowd of about 14,000.
"I'm looking forward to a great time". "It's nice that I'm feeling as good as I am and played a wonderful match against Cameron".
When it was over, Williams and Federer stood at the end of the net and took an adorable selfie together.
After much anticipation, the first set in the mixed rubber was a little disappointing, with neither Federer nor Serena returning any of the other's serves.
Earlier, Federer beat Tiafoe 6-4, 6-1 in the men's singles before Williams levelled the tie with a comeback 4-6, 6-4, 6-3 victory over Bencic.
Eight countries are competing in two groups at the tournament, which serves as a warm-up to the first Grand Slam event of the season, the Australian Open in Melbourne.
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Federer also posted the image, writing, "Oh what a night", alongside the tennis ball and fire emojis. It was super cool ... "She is a great champion and you see how focused and determined she is and I love that about her".
The two Tennis greats even had time for a selfie.
After the match, Williams told the crowd she was "kinda sad the match was over".
"He has a killer serve".
"I can't read yours either", Williams joked.
"Listen, if there's one thing I've never anxious about, it's how I would match up against Roger - because I don't think it would be very good", Williams said, laughing.
Defending champion Switzerland sealed a 2-1 victory over six-time victor United States that left it bottom of Group B but the result mattered little to over 14,000 fans in Perth and millions around the world who had eagerly awaited the contest. I watch him all the time but I never knew how great he was!' I think his serve is super underestimated, you can't read it. "I knew I could play better, so that was always something that was in my mind".