Zverev's brother Mischa Zverev was also eliminated, going out 6-3, 6-1 to David Ferrer in the first round, while Kei Nishikori eased to a 7-5, 6-2 win over Taylor Fritz to secure a place in the third round.
World number one Nadal will meet Greek teenager Stefanos Tsitsipas in Sunday's showpiece when he will target a record-extending 11th title in Barcelona a week after claiming the Monte Carlo Masters for the 11th time.
Kei Nishikori was crowned Barcelona Open champion for the second consecutive year after a hard-fought 6-4, 6-4 victory over surprise Spanish finalist Pablo Andujar on Sunday.