Qualifier Federico Delbonis of Argentina upset 14th-seeded Fernando Verdasco of Spain 6-7 (5), 7-6 (8), 6-4 to make the semi-finals of a tournament for the second time in his career.
The fifth-seeded Monaco beat Almagro in the Hamburg semi-finals last year.
Delbonis' previous semi-final appearance was two years ago, also on clay and also in Germany, in Stuttgart.
After fending off a pair of match points in the second set, Delbonis failed to serve out the match at 5-3.
He then wasted two match points on Verdasco's serve before the Spaniard double-faulted on the third.
Results from the ATP Hamburg Open tournament (x denotes seeding):
Nicolas Almagro (ESP x3) bt Juan Monaco (ARG x5) 4-6, 6-0, 6-3
Federico Delbonis (ARG) bt Fernando Verdasco (ESP x14) 6-7(5/7), 7-6(10/8), 6-4