Ferrer, the event's two-time defending champion, dispatched Lukas Lacko 6-2, 6-1 in 54 minutes. Ferrer lost only 12 points on serve and saved the only break-point chance Lacko had. He also finished off four breaks in seven receiving games.
But Monfils lost serve only twice -- despite Haas having 17 break points -- and saved all seven break points in the third set. He had 19 aces but also eight double faults.
Second-seeded Philipp Kohlschreiber recorded the only break of the match in a 7-6 (8-6), 6-4 win over Xavier Malisse and No. 4-seeded Sam Querrey came away a 6-4, 7-6 (7-5) winner over Jesse Levine in Thursday's other quarterfinals.
Monfils, who has seen his rankings slip to No. 99 18 months after reaching No. 7 in the rankings, owns a 3-0 record versus Ferrer. Querrey is 2-1 versus Kohlschreiber.