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.
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, 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.
Last year’s runners-up the United States – record 18-time competition winners – have their work cut out, trailing Belgium 2-0 in Ostend.
In Astana, hosts Kazakhstan won Saturday’s doubles rubber to lead 2023 champions Canada 2-1.
Former US Open champion Bianca Andreescu had pulled Canada level 1-1 on Friday night after Yulia Putintseva had eased past newcomer Kayla Cross in straight sets.
Anna Danilina and Zhibek Kulambayeva beat Andreescu and Cross 7-5, 6-1 in Saturday’s doubles with the tie going down to the reverse singles.