Monfils had 16 aces to set up a meeting with sixth-seeded Florian Mayer, who defeated another German, Andreas Beck, 6-4, 3-6, 6-1.
Victor Hanescu rallied to beat Dmitry Tursunov of Russia 3-6, 6-3, 7-6 (7) in 2 hours, 25 minutes.
The Romanian will next face Brazilian Thomaz Bellucci, who defeated seventh-seeded Lukas Rosol of the Czech Republic 6-4, 6-4. It was Rosol's third consecutive loss.
Nikolay Davydenko of Russia rallied to beat Spain's Daniel Munoz-De La Nava 6-7 (6), 6-3, 6-4.
Leonardo Mayer of Argentina defeated Spain's Pablo Andujar 3-6, 7-6 (5), 7-6 (5) in 2 hours, 55 minutes.
Fifth-seeded Fabio Fognini of Italy bounced back from his first-round exit at Wimbledon by defeating Britain's Alexander Ward 6-3, 7-6 (5). Germany's Nils Langer beat countryman Robin Kern 3-6, 6-4, 6-3.
Ernests Gulbis rallied past Marcel Granollers of Spain 4-6, 6-1, 6-1. The Latvian will face top-seeded Tommy Haas of Germany for a place in the quarterfinals.