The top-seeded and defending champion Melzer prevailed over Nicolas Almagro of Spain 6-4, 6-4 in the first semifinal. The win gives him his second finals appearance of the year -- he also reached the finals of the Hamburg event in July where he lost to Andrey Golubev.
Melzer owns a career mark of 2-7 in title matches.
The virtually unknown Haider-Maurer, meanwhile, followed up an upset of Marin Cilic with Saturday's semifinal victory over Germany's Michael Berrer, prevailing in a marathon 7-6 (8-6), 6-7 (1-7), 6-3, contest.
The 157th-ranked Haider-Maurer only gained entry into the main draw when Latvia's Ernests Gulbis withdrew, but he took advantage of the opportunity to become the first "lucky loser" to reach a final on the WTP Tour this year.