Monaco reached his second final in two weeks. He was runner-up in Stuttgart and is seeking his third clay-court title of the season.
The 34-year-old Haas, who has been sidelined by injuries in recent years, dropped only five points on serve in the second set. He's looking for his second title of the year after beating Roger Federer on grass in Halle last month.
Haas began the season ranked outside the Top 200, but has surged back into the Top 50 at No. 49.