With 43 Grand Slam singles titles (Williams 23, Federer 20) and more than US$200 million in prize money between them, the two 37-year-olds had welcomed the chance to face each other on the court in an official match for the first time.
It was the first time the stars had ever gone head-to-head, with Federer and Swiss partner Belinda Bencic beating USA pair Williams and Frances Tiafoe in straight sets in their Group B clash. The Swiss pair served great in the opening set, breaking Tiafoe in the fifth game after a return victor from Bencic and taking the opener after a hold at love from Roger a few minutes later.
There are two groups of four and nations play two singles and a mixed doubles match.
Williams described Federer as the greatest of all time and said his serve was underrated.
"It is very exciting for both of us and I hope a lot of tennis fans tune in and watch it", said Federer. "She is one of the biggest champions in our sport ever, men and women combined, so it is great to be playing against her". Belinda has done it in the past so I'm sure she's going to pass me on the information that I need to know about Serena's serve and all the other shots that she has.
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"I guess what you're seeking here is confirmation to see is everything there that you thought there was, or is there any fixing still to be done in the coming weeks".
"She played really well but I knew I could play better".
Two breaks of serve in each set were... Federer has 99 career titles.
The pair posed together for a selfie after the match.
"It was definitely a relief", Williams said of the result.
And the defending Hopman Cup champions now look they are marching back to the final ahead of a clash with Greece tomorrow night after a 4-2 4-3 (5-3) win in the short sets format.