Opelka upset fellow American John Isner in January at the Australian Open. Isner is the No. 1 seed this week in the indoor tournament on Long Island and plays his opening match Wednesday.
No. 6 seed Sam Querrey, who beat Mannarino in last year's semis before losing to Kevin Anderson, beat Lloyd Harris of South Africa 7-6 (2), 6-2. Brayden Schnur eased past fellow Canadian Jack Mingjie Lin, a Columbia student who earned a wild card into the tournament, 6-1, 6-3.
In other first-round matches at the Nassau Coliseum, American Ryan Harrison swept Canada's Peter Polansky 6-3, 6-4; Christopher Eubanks of the U.S. beat Adrian Menendez-Maceiras of Spain 6-4, 7-6 (4); Guillermo Garcia-Lopez of Spain ousted Australian Alexei Popyrin 7-6 (6), 6-3; and Jason Jung of Taiwan beat India's Ramkumar Ramanathan 6-3, 3-6, 6-1.