After Federer beat Frances Tiafoe 6-4, 6-1 in the men's singles, Williams overcame a sluggish start to defeat Belinda Bencic 4-6, 6-4, 6-3 and keep alive US title hopes after an opening Group B loss to Greece on Monday.
A capacity crowd of 14,064 turned out for the tie, and they saw Federer and Bencic largely capitalise on the mixed doubles inexperience of Williams' teammate, Francis Tiafoe, to clinch the deciding rubber under the Fast4 format, 4-2, 4-3 (5/3).
Federer beat Tiafoe, 6-4, 6-1, to put Switzerland 1-0 up in their group tie against the United States in the Hopman Cup.
"I used to watch him on YouTube. the rivalry between him and Rafa".
Williams leads Federer 23-20 in career grand slams won, but the two will be forever linked as the dominant male and female, respectively, in tennis for this era, maybe all-time.
"I think what I remember most about it is that we both are kind of at the pinnacle of our careers, we are both still playing some of our best tennis and that's so special to be competing against each other. I was way too excited!"
"I'll stay on the bike and keep the engine on", Federer said.
Williams herself was humble in her estimations for the upcoming match. The mother of one posted the sweet picture to her Instagram with the caption: "Goat vs Goat". It was super cool.
The U.S. lost to Greece on Monday, while Switzerland has continued its flawless start after sweeping Britain in its opener.
"It's great to be back out in match day, and it's great to be back out here", said Williams, who will be chasing a record-equalling 24th grand slam title at the Australian Open next month. "I was making a lot of errors", Williams said.
Federer was made to work during a tough first set before overwhelming the 20-year-old Tiafoe and taking control by winning seven straight games. "We try our best". He talked afterwards about the need to adjust to Perth's fast courts, and transitioning between opponents who range from a double hander, a leftie, and finally the single-hander of Tsitsipas.
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