Switzerland came through the deciding mixed doubles against Italy, with a 7-5, 4-6, 10-7 win by Belinda Bencic and Jakub Paul over Sara Errani and Andrea Vavassori, enough to see the team through 2-1 into the quarter-finals.
"There was great emotion from the team. We were into it with our hearts, and it showed," Bencic said.
Against GB, Osaka was too strong for GB reserve and 276th-ranked Katie Swan, winning 7-6(4), 6-1 as Raducanu watched from the sidelines.
"I'm proud of how I fought," Osaka said. "I'd never played here; it was tough."
GB captain Tim Henman said Raducanu, the 2021 US Open champion, could appear later in the event.
"She struggled with her foot at the end of last year. She's been practising in London and is very close. Tomorrow (against Greece) is a bit up in the air."
Billy Harris, standing in for the injured Jack Draper, began with a 7-6(4), 6-3 defeat of Shintaro Mochizuki.
"It's great to get the first win, even if there were some shaky bits. I closed it out well at the end," said Harris.
Great Britain sealed the match when Neal Skupski and Olivia Nicholls defeated Yasutaka Uchiyama and Nao Hibino 7-5, 4-6, 10-7.
Swiss veteran Stan Wawrinka, 40 and playing his final season, matched it with 23-year-old Italian Flavio Cobolli until a double-fault on match point proved to be the downfall of the three-time Grand Slam winner.
Wawrinka, playing his second three-hour match in as many days, lost 6-4, 6-7(2), 7-6 (4).
Bencic spoiled the 30th birthday of Jasmine Paolini 6-4, 6-3. "I had to convince my mind after losing to her at the end of 2025," the winner said.
"It was about mentality out there, but I knew I could do it."
In Sydney, Alexander Zverev clinched victory for Germany against the Netherlands as the world number three demolished Tallon Griekspoor 7-5, 6-0.
Eva Lys beat Suzan Lamens 6-2, 6-2, and Germany also took the doubles for a 3-0 sweep.
Playing his first match of 2026, Zverev dropped just seven points in a commanding second set.
"The first set, it was tough to get rhythm," said Zverev.
"Once I got the chances, I did well, and I'm very happy with the win."
Canada beat China, with Victoria Mboko defeating Zhu Lin 2-6, 6-2, 6-0 and Felix Auger-Aliassime overcoming Zhang Zhizhen 6-4, 6-4.
The 18-nation mixed-team event in Perth and Sydney is a warm-up for the Australian Open, which begins on January 18th.
