They're two of the greatest tennis players of all time.
The two may both be tennis superstars, but Federer admitted he barely knows Williams.
It's been billed as the most anticipated match involving men and women since 1973's "Battle of the Sexes" which saw 39-times major victor Billie Jean King beat former men's world No 1 Bobby Riggs in an exhibition match. "I used to practice with some women's players, when I was younger, you know, when I was maybe 15 and I was at the national tennis center and I used to play sets with the pro-women", said Roger Federer.
"It was great fun and she is a great champion. She is a great champion and you see how focused and determined she is and I love that about her".
Both of the legendary stars - who have a combined 43 Grand Slam titles between them - have won the Hopman Cup on two previous occasions but only Federer will have a chance at adding a third after team United States of America lost their opening two matches.
Partner Tiafoe lost his service for the Americans to lose the opening set and she also appeared to be nursing a right shoulder injury although she insisted it was of no concern with the Australian Open looming. I would even cry sometimes just because....
"I've been looking forward to it since I saw it", Williams said, per the Times.
The pair were seen at the Hopman Cup New Years Eve Gala dinner during day three of the 2019 Hopman Cup at RAC Arena on Monday (December 31) in Perth, Australia. The winners of each group will contest Saturday's final.
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. Williams and Federer served strongly and were unable to return any of each other's serves in the first set.
Roger Federer began his season with a 6-1 6-1 thrashing of Great Britain's Cameron Norrie at the Hopman Cup in Perth.
"It's definitely a relief".
Serving at 4-2, Serena fends off two break points and bringing the game home with an ace before a solid hold in the ninth game that sent her over the finish line and kept the US in contention.
"I admire everything she's done on and off the court". Federer has 20, Williams 23.
Meanwhile in regular WTA action, Serena's elder sister Venus showed she is still a force by beating Victoria Azarenka 6-3 1-6 6-3 in a battle of former world number one's at the Auckland Classic. "I wasn't missing any rhythm", Federer said after his win over world No.91 Norrie.