Tim Puetz and Jan-Lennard Struff stunned Marc Lopez and Feliciano Lopez 6-3, 6-4, 3-6, 6-7 (4/7), 7-5 as Spain were left to rue converting just two of 20 break points in a four-hour 40-minute encounter at the Plaza de Toros bullring.
Nadal, 31, who marked his comeback on Friday after more than two months on the sidelines with a hip injury by seeing off Philipp Kohlscreiber in his opening singles match, now has to repeat his magic against Zverev, 11 years his junior.
Nadal is on a record run of 23 successive wins in the Davis Cup, having not lost since 2005.
Five-time champions Spain, meanwhile, have won 26 straight home ties, with their last defeat on Spanish soil against Brazil in 1999.
"I have complete faith in all my players, they're all formidable," said Spain captain Sergi Bruguera.
Despite the pressure, Nadal can look ahead to his meeting against Zverev with confidence as he boasts a 3-0 record over the beanpole German including their only meeting on clay in Monte Carlo last year.
Germany captain Michael Kohlmann hailed his doubles team.
"This is what the Davis Cup is all about. It was 50/50 in the end. We got a bit lucky but it feels great," he said.
In Genoa, Nicolas Mahut and Pierre-Hugues Herbert put defending champions France 2-1 up against Italy with a 6-4, 6-3, 6-1 win over Fabio Fognini and Simone Bolelli.
It took two-time major winners Mahut and Herbert just 1 hour 54 minutes to take revenge for a loss to the Italian duo in the 2015 Australian Open final.
Lucas Pouille, the world number 11, will have the chance to clinch a semi-final spot for France for the third time in four years when he faces Fognini in the first of Sunday's reverse singles.
"Pierre-Hugues and Nico played the perfect match," said France captain Yannick Noah. "Now we are going to prepare Lucas to beat Fabio -- we will give 150 percent for this match."
The winner of the tie will face either Spain or Germany for a place in the final.
Croatia, the 2005 champions, are also one win away from the last-four after Ivan Dodig and Nikola Mektic defeated the Kazakh pair of Timur Khabibulin and Aleksandr Nedovyesov 6-7 (2/7), 6-4, 6-4, 6-2.
"We played together for the first time and that's why we needed some time to find our game," said Dodig.
On Sunday, world number three and former US Open champion Marin Cilic can wrap up the tie when he faces Mikhail Kukushkin, the world 92 in the first of the reverse singles.
Kukushkin stunned world number 28 Borna Coric in Friday's singles.
In Nashville, the United States became the first country to punch their ticket to the Davis Cup semi-finals on Saturday following a doubles win by Jack Sock and Ryan Harrison.
The pair had to work for the victory on the hardcourts which gave the Americans an insurmountable 3-0 lead and a berth in the semis for the first time since 2012.
Sock and Harrison needed three hours to dispatch Belgian debutants Sander Gille and Joran Vliegen in four sets 5-7, 7-6 (7/1), 7-6 (7/3), 6-4 in front of a crowd of 5,000 at the Curb Event Center.
John Isner and Sam Querrey lifted the United States into a 2-0 lead over last year's runners-up Belgium on Friday.
Isner needed three hours and 14 minutes to dispose of world number 319 Joris de Loore 6-3, 6-7 (4/7), 7-6 (10/8), 6-4, while world number 14 Querrey beat 110th ranked Ruben Bemelmans 6-1, 7-6 (7/5), 7-5.
By AFP's Dave James