Radu Albot of Moldova upset fifth-seeded Steve Johnson in straight sets in the first round of the Stockholm on Monday.
Albot held his serve throughout the match to defeat the American 6-3, 6-4.
Eighth-seeded Marcos Baghdatis also got an early exit as South Africa's Kevin Anderson beat the Cypriot in three sets, 2-6, 6-4, 6-2.
In other first-round action, Adrian Mannarino of France beat Donald Young of the United States 7-5, 6-4, and American Ryan Harrison defeated Adam Pavlasek of Czech Republic 6-3, 7-5.
Top-seeded Gael Monfils and second-seeded Grigor Dimitrov enter the tournament later in the week.