After the opening match win for Roger Federer over Frances Tiafoe, Switzerland and the United States of America played much tighter encounters in the rest of the tie, with Switzerland earning a 2-1 win to stay undefeated and continue their march towards the place in the final.
Williams will take on Belinda Bencic in the women's singles rubber.
Williams echoed those sentiments, calling the match "a dream come true" even as she expressed relief that she wasn't facing Federer in a singles match.
The tournament format involves two singles matches and a mixed doubles meeting between countries in a round-robin group stage, before the winners of the two groups meet in the final on Saturday. "Maybe one of the toughest first round matches I have played", she said.
"I see why they talk about [her serve] - you just can't read it", Federer said.
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.
For the first time in their decades-long tennis careers, Serena Williams and Roger Federer will play each other on New Year's Day. His serve is super underestimated, he has a killer serve. "She is a great champion and you see how focused and determined she is and I love that about her".More news: Partial Federal Shutdown Into Ninth Day With Trump Blaming Democrats
More news: Floyd Mayweather Destroys Tenshin Nasukawa In One Round
More news: Florida's minimum wage rises to $8.46 in 2019
Roger Federer gave notice that he is in good shape before his attempt to win a third consecutive Australian Open title with a comprehensive victory at the Hopman Cup in Perth. In his press conference at the Hopman cup, Roger Federer spoke about his experiences of playing against, and with, quality women tennis players.
Switzerland will defend its unbeaten record against Greece in its final round robin match Wednesday with the U.S. taking on Britain.
"She played really well but I knew I could play better".
The pair posed together for a selfie after the match. Belinda won one point more than Serena but she couldn't do enough in the crucial points, suffering three breaks from as many chances and stealing Serena's serve twice from five opportunities.
"It's the age of social media", Federer said with a laugh when told about Williams' plan.