In Sunday's final, Nalbandian will play Robin Soderling or teenager Kei Nishikori.
Nalbandian, who has reached two other finals this year, dominated the third-seeded Finn from the start, breaking his serve in the opening game.
Nieminen, a two-time finalist at the Stockholm Open, broke back but Nalbandian broke twice in to take command. Serving for the set, Nalbandian fell behind 15-40, then scored four straight points.
Despite leading 40-15, Nieminen dropped his serve again in the third game of the second set.
Nalbandian broke for a 4-1 lead and held to lead 5-1. Nieminen double-faulted on match point.
Nalbandian is 1-1 in finals this year, winning in Buenos Aires in February and losing in Acapulco the next week.
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