Four-time Grand Slam champion and former world number one Osaka last played for Japan in the competition in 2020, before becoming a mother.
In Tokyo, the 26-year-old defeated Yulia Putintseva 6-2, 7-6 to give Japan a commanding lead over Kazakhstan after Nao Hibino had beaten Anna Danilina 6-1, 6-0 earlier in their qualifying clash. The Asian nation went on to secure the tie and a spot in the finals.
Osaka said she was "super-nervous" about playing in Japan for the first time since appearing at the Pan Pacific Open in September 2022.
"I'm the kind of person that I don't like to disappoint people," said Osaka who sent down 15 aces.
"It would probably devastate me a lot more to lose here than it would to lose in a regular tournament, just because I want to support everyone as much as they support me."
Osaka, whose world ranking has risen from 831 to 193 since she began her comeback this year, has not given up hope of making the Paris Olympics later this year. However, she may need to go through an appeals process after failing to make a mandatory two appearances in the BJK Cup during the current Olympic cycle.
World number one Swiatek, back on national duty for the first time since 2022, eased past 158th-ranked Simona Waltert 6-3, 6-1 to give Poland the lead over Switzerland in Biel.
Magdalena Frech made it 2-0 with a come-from-behind 6-7, 7-5, 6-3 victory over 18-year-old Celine Naef.
Switzerland were 2022 champions when they defeated Australia in the final inspired by Belinda Bencic. However, Olympic champion Bencic is currently off tour after announcing in November last year that she was pregnant.
'Quite embarrassing'
France, the three-time champions, are level 1-1 with Great Britain in Le Portel.
World number 49 Diane Parry gave France the lead with a surprisingly easy 6-2, 6-0 win over 28th-ranked Katie Boulter.
Raducanu, back in the side after an injury-plagued season, pulled Britain level with a 3-6, 6-3, 6-2 win over French number one Caroline Garcia.
Raducanu admitted to a moment of embarrassment at 5-1 in the decider when she mistakenly thought she had won the match.
"I think sometimes I'm just super locked in that I don't get the score," she explained.
"I'm not sure what happened, but it was quite embarrassing. All I was thinking was, 'if I lose this right now I'm going to look like a right muppet'. So I'm very happy that I managed to pull it through in the next game."
Australia surged into the finals with a 4-0 victory over Mexico, handing Grand Slam champion Samantha Stosur a winning start to her reign as captain.
The seven-time champions took a commanding 2-0 lead into day two at Brisbane's Pat Rafter Arena, allowing Stosur to rest Arina Rodionova and hand a debut to 18-year-old Taylah Preston.
She seized the opportunity with both hands, sweeping past the experienced Marcela Zacarias, 12 years her senior, 6-1, 6-1 to put the tie beyond Mexico's reach.
Daria Saville and Ellen Perez rubbed salt in the wounds by beating Mexican duo Jessica Hinojosa Gomez and Maria Fernanda Navarro 6-3, 6-1 in the dead rubber doubles.
Ukraine eye first finals
War-torn Ukraine are playing their home tie against Romania in Florida and their attempt to reach the finals for the first time enjoyed a flying start.
Lesia Tsurenko fought back to beat Ana Bogdan 3-6, 6-2, 6-0 before Elina Svitolina saw off Jaqueline Cristian 6-3, 7-5.
"I was very nervous because I really wanted to get points for my team," said Tsurenko after her match which saw 16 breaks of serve.
Slovakia lead Slovenia 2-0 in Bratislava.
The eight winners this weekend advance to November's 12-team finals in Seville where they join hosts Spain, defending champions Canada, 2023 runners-up Italy and wild-card Czech Republic.
