But the fact it would also decide the tie victor added extra intrigue. The United States, which lost to Greece on Monday, can't now advance.
Only the group winners advance to the final, so the closing doubles could ultimately decide the victor of a group that also contains Greece and Britain.
After the interview, they even posed for a selfie.
But the dream quickly turned into a nightmare as Federer turned the screws on the unheralded Brit.
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.
"It was so fun. After all these years, we've actually never done this, so it was super cool that we get to do it at such a pivotal point in both our careers", Williams added.
"This guy (Federer) is great, both off the court and on the court". He has a killer serve, - literally, you can't read it.
Earlier in the night, Williams raced out to a 3-0 lead in the opening set of her singles match, but Bencic won six of the next seven games to put her 37-year-old opponent on the back foot.
Tickets were sold out soon after being released for the showpiece encounter, which pits Federer and Williams on court for the first time.
"It's definitely a relief".
The much-hyped contest quickly lived up to its billing with Federer nearly running down Williams' smash into the open court. He came up for the plan that was for the moment, but also longevity, and I did really well there.
Federer faced three break points in his first service game, but was near flawless from that point on as he wrapped up the contest in 57 minutes at RAC Arena on Sunday night.
Switzerland have two wins from two games ahead of Thursday's meeting with Greece, while US take on Britain next. He discovered his groove by dominating Norrie's serve and broke in the second game with a powerful cross-court backhand victor.