Qualifier Oscar Otte knocked out No.7 seed Jan-Lennard Struff 6-4, 6-3 in an all-German matchup during the first round of the bett1HULKS Indoors in Cologne.
Steve Johnson also won a first-round upset, beating fifth-seeded Filip Krajinovic of Serbia 3-6, 6-4, 6-4.
Sixth-seed Hubert Hurkacz of Poland, Austrian Dennis Novak, Radu Albot of Moldova and German wild-card entrant Mischa Zverev all progressed.
In Sardinia wild-card Lorenzo Musetti beat No.8 seed Pablo Cuevas of Uruguay 7-6 (4), 7-5 in day one of the event at Santa Margherita di Pula, Sardinia.
Musetti, 18, will meet Italian qualifier Andrea Pellegrino in the next round after his opponent Stefano Travaglia retired while trailing 5-6, 7-6 (7), 3-0.
NO.7 seed, American Tommy Paul, and Czech Jiri Vesely both won their openers.