Qualifiers Marius Copil and Jozef Kovalik beat higher-ranked opponents to begin their European Open campaigns in style.
In the maiden staging of the event, it was Copil who sprung the biggest surprise with his 4-6 6-3 6-3 win over sixth seed Joao Sousa.
A huge 165-place rankings gap did not daunt the Romanian, whose fellow qualifier Kovalik overcame Guillermo Garcia-Lopez 6-3 6-3.
Great Britain's Kye Edmund needed two tie-breaks to oust Illya Marchenko 7-6 (8-7) 7-6 (7-5) and set up a second-round tie with second seed David Ferrer.
Also on Tuesday, seventh seed Nicolas Mahut beat Diego Schwartzman 7-6 (7-2) 7-5, and there were wins for Jan-Lennard Struff and Florian Mayer.