The fifth-seeded Garcia-Lopez of Spain rallied past German Dustin Brown 6-7 (5/7), 6-2, 6-2 at the $600,000 hardcourt event, a final US Open tune-up.
Nieminen, the No. 15 seed, beat German Benjamin Becker 7-5, 6-4.
American Sam Querrey defeated Spain's Pere Riba 7-6 (7/5), 6-4 in first-round action. Querrey will next face fellow American Steve Johnson on Tuesday.
In other opening-round action, Frenchman Paul-Henri Mathieu toppled American wild card Robby Ginepri 6-3, 7-5, Poland's Jerzy Janowicz rolled over Argentine Carlos Berlocq 6-1, 6-4, and Kitzbuhel champion David Goffin of Belgium ousted Austrian veteran and defending Winston-Salem champ Jurgen Melzer 6-3, 6-4. Melzer beat Frenchman Gael Monfils in last year's final.
Goffin has won his last 23 matches, which includes three consecutive titles on the Challenger Tour.
He has gone unbeaten since losing in the first round at Wimbledon to Andy Murray.
"The confidence is really high at the moment," said Goffin. "I'm feeling so good. I played really well this summer, and now I keep winning, so for the moment it’s a great period for me."
Canada's Frank Dancevic, Dutchman Igor Sijsling, Argentine Federico Delbonis, Frenchman Adrian Mannarino, Blaz Rola of Slovenia and Ukrainian Aleksandr Nedovyesov also posted first round victories.
This week's top seed is two-time champion John Isner of the US.