Andreea Mitu of Romania upset world No.7 Lucie Safarova of Czech Republic 6-3 6-4 in the opening round of the Generali Ladies on Tuesday.
French Open finalist Safarova, who was the top-seeded player at the Austrian indoor event, went 3-0 down in the opening set after winning just four of the first 16 points.
She dropped her serve again at 3-3 in the second set.
"I didn't expect it, but I believed I could make a good match," said the 95th-ranked Mitu, who will next play Anna-Lena Friedsam of Germany.
"It's good to have such a tough match, as it makes you stronger for the rest of the tournament."
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
Last year's runner-up Camila Giorgi defeated Julia Goerges of Germany 6-3 6-4 in a match between two former finalists.
The sixth-seeded Italian, who also beat Goerges here in straight sets two years ago, saved all six break points against her serve.
Fifth-seeded Anna Karolina Schmiedlova of Slovakia pulled out because of a viral illness and was replaced by Johanna Konta of Britain, who defeated Anna Beck of Germany 6-1 6-3.
In other first-round play, Madison Brengle of the United States beat Misaki Doi of Japan 6-0 6-3; Alison van Uytvanck of Belgium defeated Lucie Hradecka of the Czech Republic 6-2 7-5; and Stefanie Voegele of Switzerland beat Austrian wildcard entry Tamira Paszek 6-4 7-5.
