Olympic champions Sara Errani and Jasmine Paolini sealed the winning point with a 6-2, 7-5 win over Japan's Shuko Aoyama and Eri Hozumi on clay at Velletri, south of Rome.
Paolini and Elisabetta Cocciaretto had given the hosts a 2-0 overnight advantage.
Later Saturday, Japan earned a consolation win when Himeno Sakatsume defeated Lucia Bronzetti 7-6(4) 6-2.
Last year's runners-up, the United States - record 18-time competition winners - dragged themselves back into the tie with Caty McNally and Nicole Melichar's doubles win.
Their 6-4, 6-2 success over Magali Kempen and Elise Mertens earned a vital point for the Americans, with Belgium leading 2-1 in Ostend.
The US now have to win Saturday's two reverse singles rubbers to reach September's eight-team finals in Shenzhen, China.
In Melbourne, Harriet Dart and Jodie Burrage won the doubles rubber to power Britain past Australia.
Later, Emerson Jones pulled a point back for Australia for 3-1 after beating Katie Swan 7-5, 6-3 in Saturday's reverse singles.
Britain started the day with a commanding 2-0 lead after rising star Mika Stojsavljevic upset Talia Gibson, and Dart beat Kimberly Birrell on day one.
It left Australia's Storm Hunter and Ellen Perez facing a must-win clash in Melbourne, but they failed to rise to the occasion and crashed 6-3, 6-4.
"That feels amazing," said an emotional Burrage, who has battled injuries in recent times.
"I'm proud of us for the way that we played, the way that we came out and managed to close that out."
Victory propelled Britain - semi-finalists last year - into the September eight-team finals in Shenzhen, China.
Hunter and Perez broke early for a 3-1 lead, but they imploded with Dart and Burrage reeling off the next five games to claim the opening set.
A seesawing second set saw four service breaks before the British pair broke again for a 5-4 advantage, and Burrage made no mistakes in serving out for the win.
It was an impressive victory with none of Britain's top four making the long trip with Emma Raducanu, Katie Boulter and Fran Jones opting to focus on the European clay-court season, and Sonay Kartal hurt.
"They're two great players," said Dart of Hunter and Perez. "They play a lot of doubles, and it was our first time playing together.
"It's crazy actually, because we played pretty well and to be able to make the finals is just kind of surreal."
Seven ties are being played this weekend.
Ukraine, Spain & Kazakhstan seal berths
Ukraine, Spain and Kazakhstan also sealed their tickets to the final.
Twins Lyudmyla and Nadiia Kichenok gave Ukraine an unassailable 3-0 lead after the doubles in the southern Polish city of Gliwice.
Marta Kostyuk and Elina Svitolina won Friday's singles.
Overnight, Slovenia and Spain had been tied 1-1 on clay in Portoroz.
But Aliona Bolsova and Sara Sorribes Tormo teamed up to win the doubles. Kaitlin Quevedo then rallied past Veronika Erjavec 7-6(4), 6-2 to seal the winning point.
Kazakhstan won 3-1 against Canada after former US Open champion Bianca Andreescu lost her reverse singles tie 7-6(5), 3-6, 7-6(4) to Yulia Putintseva.
Andreescu had pulled Canada level 1-1 on Friday.
But Anna Danilina and Zhibek Kulambayeva beat Andreescu and Cross 7-5, 6-1 in Saturday's doubles.