It was a bad day for Germany's Zverev siblings who lost semi-finals in different countries.
Tomas Berdych has become the latest seeded player to crash out of the Stuttgart Open, with Spaniard Feliciano Lopez edging past the Czech in three sets to reach the last four.
Lopez saved all nine break points he faced against the third-seeded Berdych.
A victory for Lopez would bring him the third grass court success of his career, following victories at Eastbourne in 2013 and 2014.
The 35-year-old reigned supreme as he won 6-7 (4), 6-3, 6-4 to book a place in the semi-final on Friday.
Zverev, the sixth seed, appeared the stronger server and took the first set tie-break as Lopez found the net too often.
Also, the fourth-seeded Lucas Pouille defeated Philipp Kohlschreiber 6-4, 2-6, 6-3 for a semifinal against French compatriot Benoit Paire, who beat Poland's Jerzy Janowicz 6-1, 7-6 (4). In the tie break, Zverev scored 5 mini-breaks to grab it by 7-2, and he was just a set away from his first ATP final on grass.
In the women's event in the Netherlands, Russian Natalia Vikhlyantseva will meet Anett Kontaveit of Estonia in Sunday's final.