Monaco is seeded third and is the highest-ranked player left. But the Argentine needed nearly two hours to beat his Argentine compatriot, a left-hander who is ranked 101st and was playing in his first semifinal on the tour.
Pella saved two match points and wasted one set point before sending a backhand into the net to end the baseline duel. The 19th-ranked Monaco finished with eight aces to Pella's 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. 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 the Finn recovered to win the next five games and served out the match on his second match point when Sijsling's forehand sailed long.
The clay-court tournament replaced the World Team Cup and is a warmup for the French Open. The year's second Grand Slam event starts Sunday.