Monaco reached the final by beating Guido Pella 6-4, 7-6 (8) Friday. Nieminen defeated Igor Sijsling of the Netherlands 7-5, 4-6, 6-3.
Monaco is seeded third and the highest-ranked player left in the field. But he needed nearly two hours to overcome his left-handed, 101th-ranked opponent, who was playing in his first semifinal match on the tour after going through qualifying.
Pella saved two match points — and wasted one set point — before whipping a backhand into the net to end the grueling battle from the baseline between two Argentines. The 19th-ranked Monaco finished with eight aces, while Pella had only one.
"It's my first final this year, so I'm very happy," Monaco said. "I played a couple of good matches, so I've got good rhythm. We played a great match (today). We've known each other for a long time and have had many practices together.
"In the second set especially, we fought until the last point."
The sixth-seeded Nieminen fell behind 2-0 in the third set and faced two break points. But he recovered to win the next five games and served out the match on his second match point, when Sijsling's forehand sailed long.
Nieminen holds a 2-1 head-to-head record against Monaco.
The clay-court tournament is a warm-up for the French Open, the second Grand Slam of the year that starts on Sunday. The event replaced the World Team Cup.
Results from the sixth day of the Dusseldorf ATP tournament (x denotes seeding):
Semi-finals:
Juan Monaco (ARG x3) bt Guido Pella (ARG) 6-4, 7-6 (10/8)
Jarkko Nieminen (FIN x6) bt Igor Sijsling (NED) 7-5, 4-6, 6-3