Wild card De Voest kicked off the tournament, which is being played at Montecasino, with a hard-fought victory over Italian qualifier Stefano Galvani in the first match of the main singles draw.
The 30-year-old won 6-3 5-7 7-6 (8/6) in an encounter that lasted two hours and 40 minutes.
The players were evenly matched throughout the clash, but it was ultimately De Voest's first serves that saw him through to the second round, where he will meet either Austrian Martin Fischer or eighth-seeded Pole Michal Przysiezny.
Devvarman, the world number 110, was the highest-ranked player in action and he came through relatively easily against another local. He saw off qualifier Raven Klaasen 6-4 6-3.
The Commonwealth Games champion was a quarter-finalist in the competition last year and looked untroubled in his first match back.
He was especially ruthless on his serve as his opponent failed to break him in what was the second of the evening matches.
In the earlier clash, Greul needed just 77 minutes to dispatch Brazilian Thiago Alves in straight sets 6-2 7-5.
Next up for the Stuttgart-based player is a meeting with second seed Yen-Hsun Lu or little-known Canadian Milos Raonic.