Five-time champions Spain are defending a run of 23 consecutive home victories in the tournament, a streak stretching back to 1999.
World number five David Ferrer beat Sam Querrey 4-6, 6-2, 6-2, 6-4 before Nicolas Almagro won a four-hour slog against giant American John Isner, 6-4, 4-6, 6-4, 3-6, 7-5.
"It was a hard match, difficult at every moment," said 30-year-old Ferrer.
Isner saved three match points in the ninth game of the decider but buckled on a fourth when he went long with a weary forehand in the 12th game.
"That was really disappointing because I did such a good job the whole fourth and fifth set of just hanging in there and I just kind of gave it away at the end," Isner said.
In Buenos Aires, Argentina, who have yet to win the tournament, and were defeated by Spain in the 2008 and 2011 finals, were 1-1 in the other semi-final against the Czech Republic.
Tomas Berdych recovered from two sets to one down, and 1-4 behind in the fourth set, to stun Juan Monaco 6-1, 4-6, 1-6, 6-4, 6-4 and pull the Czechs level.
With Argentine football legend Diego Maradona and former tennis glamour girl Gabriela Sabatini amongst the raucous 13,00-crowd inside Parque Roca, Berdych dug deep to claim a famous win.
Berdych, who defeated Roger Federer on his way to the US Open semi-finals last week, won nine straight games to lead 5-3 in the decider.
But Monaco stopped the rot for a break to trail 4-5 before Berdych broke again to seal victory.
Earlier, world number eight Juan Martin del Potro beat Radek Stepanek 6-4, 6-4, 6-2 to earn a comfortable first point.
Del Potro was playing against medical advice after hurting his wrist at the US Open and he broke down in tears as he gave his victory speech. "It was very difficult to play with a hurt wrist," said Del Potro. "But the crowd was fantastic. I am very passionate about playing for my country. I like to give them a good show."
In the play-offs, where a place in the elite World Group will be assured for 2013, world number one Roger Federer and his 2008 Olympic gold medal partner Stanislas Wawrinka put Switzerland 2-0 up against the Netherlands in damp and chilly Amsterdam.
Federer eased past Thiemo de Bakker 6-3, 6-4, 6-4 before Wawrinka defeated 50th-ranked Robin Haase 6-3, 3-6, 6-3, 7-6 (7/4).
"It was important to get off to a good start and I played solidly in rainy and windy conditions," said Federer.
In Hamburg, Florian Mayer saw off Lleyton Hewitt 7-5, 6-3, 6-2 as Germany finished the day at 1-1 against Australia.
Hewitt became Australia's most capped player with 33 Davis Cup appearances, but the 31-year-old, now ranked 100 in the world, was swept aside by 25th-ranked Mayer.
Bernard Tomic had put Australia ahead with a 2-6, 6-3, 6-4, 7-6 (7/4) victory over left-hander Cedrik-Marcel Stebe.
Belgium are one win away from ending Sweden's 12-year stay in the World Group after opening a 2-0 lead in Brussels where Steve Darcis beat Michael Ryderstedt 6-3, 7-6 (7/3), 6-0 and David Goffin defeated Andreas Vinciguerra 6-4, 6-3, 7-5.
In Naples, Andreas Seppi beat Guillermo Hormazabal 7-5, 6-1, 6-2 to give Italy a 1-0 lead over Chile before bad light halted the second rubber between Fabio Fognini and Paul Capdeville at 1-1 in the final set.
Japan and Israel were level at 1-1 in Tokyo with Go Soeda beating Dudi Sela 6-2, 6-4, 3-6, 6-4 and Amir Weintraub seeing off Tatsuma Ito 6-3, 6-2, 6-4.
Ito was only playing because Kei Nishikori suffered a shoulder injury on Friday morning.
In Astana, Kazakhstan and Uzbekistan were level at 1-1.
Canada were 2-0 up on South Africa in Montreal while Brazil took a 2-0 lead over two-time champions Russia in San Jose do Rio Preto.