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Vienna, Austria (Sports Network) - Top seed Juan Martin del Potro advanced to his second straight final at the Erste Bank Open on Saturday with a semifinal win over Gilles Muller, but second-seeded Janko Tipsarevic was upset by unheralded Grega Zemlja of Slovenia.

Del Potro battled through a pair of tiebreaks, 7-6 (7-4), 7-6 (7-5), to beat Muller and reach the Vienna final again. The Argentine will try to finish with a win this year, though, having lost to Jo-Wilfried Tsonga in last year's title match.

Zemlja continued his surprising run through the draw with a 3-6, 7-6 (7-5), 6-2 win over Tipsarevic, who lost in the Vienna semifinals for the second time. The Serb also reached the round of four in 2009.

The loss damaged Tipsarevic's chances for a spot in the season-ending ATP World Tour Finals. He entered this week ninth in the standings with only the top eight qualifying for the lucrative tournament.

Zemlja first had to qualify for the main draw this week and was playing in his first ATP World Tour semifinal after beating a pair of seeds -- No. 8 Xavier Malisse and No. 3 Tommy Haas.

Saturday's stunning win was his first in five tries over a top-10 opponent. One of those four losses came against Tipsarevic in the third round of this year's U.S. Open.

While Zemlja will be trying for his first ATP Tour title, del Potro will be gunning for his third of 2012 and the 12th of his career. The former U.S. Open champ won in Marseille and Estoril, while also losing in the Rotterdam final to Roger Federer. He was also the bronze medalist at the London Olympics.

Zemlja and del Potro will meet for the first time on Sunday.

The champion will pocket $113,795.

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