Austria also joined hosts Italy in the culminating round set for November with a dramatic 3-2 win over Hungary, while Belgium and Denmark enjoy commanding 2-0 leads over Australia and Spain.
Argentina's Andres Molteni and Horacio Zeballos beat Dutch duo Sander Arends and Botic van de Zandschulp 6-3 7-5 to take an unassailable 3-0 lead in Groningen.
After last year's semi-finalists Germany went up 2-0 on Friday in Tokyo, their victory was completed on Saturday when Kevin Krawietz and Tim Puetz saved three set points in the second set to beat Japan's Yosuke Watanuki and Takeru Yuzuki 6-3 7-6 (7-4) in the doubles. Justin Engel then beat Rei Sakamoto.
"Right now, the locker room is dancing and singing. We're looking forward to Bologna and hopefully we're getting one step further," Germany captain Michael Kohlmann said.
Hungary, who had lost both their matches to Austria on Friday, responded with back-to-back wins in Debrecen.
Fabian Marozsan and Zsombor Piros edged the Austrian pair of Alexander Erler and Lucas Miedler 7-6 (7-4) 7-6 (7-2) before Piros beat Lukas Neumayer in a singles match to bring Hungary level at 2-2.
But Austria won the tie in the fifth match, with Jurij Rodionov earning a dominant 6-2 6-1 victory over Marton Fucsovics.
France also secured their place in the Final 8 as Corentin Moutet beat Marin Cilic, securing a 3-1 victory over Croatia in Osijek.
Last year's quarter-finalists Spain are on the verge of elimination as Denmark took a 2-0 lead over them in Marbella, with world No.11 Holger Rune beating Pablo Carreno Busta and Elmer Moller defeating Jaume Munar.
Australia's Alex de Minaur, who reached the US Open quarter-finals earlier this month, lost to Belgium's Raphael Collignon 7-5 3-6 6-3 in Sydney, and Zizou Bergs beat Jordan Thompson in straight sets to consolidate the visitors' 2-0 lead.
