The 72nd-ranked Haase, who has won both his career titles in Austria, lost serve in the opening game but had two breaks to take the first set.
The eighth-seeded Thiem, whose breakthrough season has seen him rise to No. 38, leveled the score with one break in the second but failed to recover from dropping 4-1 behind in the final set.
"I was just too weak today," the 38th-ranked Thiem said.

"It's bitter to lose in front of your home crowd but Robin played very solid. He hasn't got the fastest serve but I couldn't read them and missed a lot of returns,” he added.
Haase, who was beaten for the title by Tommy Haas last year, next takes on Benjamin Becker of Germany, who rallied to defeat Victor Hanescu of Romania 4-6, 6-1, 6-3.
Two-time former champion Jurgen Melzer came from a set and a service break down to defeat Norbert Gombos of Slovakia 3-6, 6-4, 6-3.
The Austrian, a former No. 8 who won the event in 2009 and 2010, was broken in the opening game of the second set by Gombos, who was making his ATP Tour debut. But Melzer gained control of the match after immediately breaking back.
"This is the kind of win I need to build self-confidence," said Melzer, who had a seven-month injury layoff earlier this season and has slipped to 123th in the rankings.

"He put so much pressure on me in the first set. I knew he wouldn't be able to keep that up otherwise he would have been a top-20 player,” he said.
Melzer next plays Ivo Karlovic of Croatia, who saved three match points in the second set before beating Federico Delbonis of Argentina 6-7 (1), 7-6 (9), 7-6 (2).
The seventh-seeded Karlovic served 29 aces in a match that went to three tiebreakers and had no break points.

Vasek Pospisil of Canada edged Daniel Brands of Germany 6-3, 6-7 (3), 7-6 (2) to set up a second-round match against Andy Murray, who had a bye in the opening round.
In other first-round play, Jan-Lennard Struff of Germany defeated seventh-seeded Guillermo Garcia-Lopez of Spain 6-3, 6-4; Viktor Troicki of Serbia had 14 aces and conceded only five points on serve while beating Victor Estrella Burgos of the Dominican Republic 6-0, 6-3; and Tomaz Bellucci of Brazil downed Paul-Henri Matthieu of France 6-1, 6-4.

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