The 19-year-old Mannarino -- a native of Soizy, France, and the world's 181st-ranked player -- beat fellow Frenchman Marc Gicquel 6-4, 7-6 (7-3) in a match watched by a divided crowd on the indoor hard court at Metz's Les Arenes.
France's Paul-Henri Mathieu, the fourth seed, downed unseeded Serbian Janko Tipsarevic 7-6 (7-4), 6-3; fifth-seeded Dmitry Tursunov of Russia, a four-time champion at this tournament, beat former world No. 1 Carlos Moya of Spain 6-3, 6-3; and seventh-seeded Radek Stepanek of the Czech Republic rallied to beat unseeded Eduardo Schwank of Argentina 3-6, 7-6 (7-4), 6-4.
In Saturday's semifinals, Mathieu faces Mannarino and Tursunov plays Stepanek.
The eventual champion will earn $85,000 for winning Sunday's title match.