Nadal's 6-1, 6-4, 6-4 win in Valencia draws the tie level at 2-2, with Spanish world number 33 David Ferrer up against Germany's Philipp Kohlschreiber in the fifth and decisive match.
Two breaks in each set were enough for Nadal to seal an impressively comfortable victory over Zverev in what was only the 31-year-old's second outing since returning from a hip injury.
Nadal was forced to retire at the Australian Open in January but, back on his favoured clay, the 16-time grand slam champion showed little sign of rust against Zverev, whom he has now beaten four times out of four.
It was also the 24th consecutive match Nadal has won in Davis Cup.
He beat Kohlschreiber in straight sets on Friday after Zverev had overcome Ferrer. Germany then took a 2-1 lead when Tim Puetz and Jan-Lennard Struff won the doubles rubber against Spain's Feliciano and Marc Lopez.