Four-time Grand Slam champion Naomi Osaka got her 2026 season off to a winning start for Japan in the United Cup after Great Britain's Emma Raducanu pulled out of their singles clash with a fitness issue.
But despite Osaka's win, GB took the match 2-1 with a deciding mixed doubles victory.
Osaka was too strong for GB reserve and 276th-ranked Katie Swan, winning 7-6 (7/4), 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.
"I don't think she's necessarily out of this tournament just yet. So we're keeping our fingers crossed," said Henman.
Billy Harris, standing in for the injured Jack Draper, began with a 7-6 (7/4), 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.
In Sydney, Alexander Zverev clinched victory for Germany against The Netherlands as the world No 3 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."
The 18-nation mixed-team event in Perth and Sydney is a warm-up for the Australian Open which begins on January 18.