Pella pulled within one victory for his first ATP finals appearance with a 7-6 (7-5), 7-5 win over seventh-seeded Viktor Troicki. Pella ousted top-seeded Janko Tipsarevic in the second round.
Pella pulled off six breaks against Troicki, with four coming in the second set. He won as many points (43-43) when Troicki served as Troicki managed.
He'll need a third consecutive win over a seeded player to reach his first ATP final as third-seeded Juan Monaco defeated Tobias Kamke 6-3, 6-4 in just more than an hour. Monaco converted five of his six break chances in the match.
The fourth semifinal slot went to Igor Sijsling, who routed Jan Hajek 6-3, 6-1 in 57 minutes. Sijsling fought off all six of the break points he faced while taking about 75 percent of the points on serve.
Monaco and Pella have never before met in an ATP event. Nieminen has won two of his three matches against Sijsling, including a three-set battle in California earlier this year.