Del Potro beat Muller 7-6 (7-4), 7-6 (7-4) to earn a berth in Sunday's championship match against qualifier Grega Zemlja of Slovenia, who upset second-seeded Janko Tipsarevic in the other semifinal match Saturday.
It will be the second consecutive year Del Potro has reached the Erste Bank finals. He lost last year to Jo-Wilfried Tsonga.
Zemlja, who ousted No. 3 seed Tommy Haas earlier in the tournament, kept his hot streak going with a 3-6, 7-6 (7-5), 6-2 victory over Tipsarevic, who lost in the Vienna semifinals for the second year in a row.
The loss could cost Tipsarevic a trip to the ATP World Tour Finals. He began the week ranked ninth in the world and the finals field is limited to the top eight players.
Swim Week Miami Beach 2014 [PHOTOS]