Kitzbuhel - Austrian veteran Jurgen Melzer defeated a top 10 opponent for the first time in five years on Wednesday with a 6-3, 7-5 victory over top seeded compatriot Dominic Thiem at Kitzbuhel and will face his younger brother Gerald in the quarter-finals.
The 35-year-old is competing in just his second event since last year's US Open after undergoing left shoulder surgery in November.
His last Top 10 win came in 2011 against Roger Federer in the quarter-finals of the Monte-Carlo Masters.
"As a sportsman, you need to believe in your chances to win. Today this chance was very little, but I tried hard to use it," Melzer told the ATP World Tour website.
"I think I showed almost perfect tennis, which one needs to play when facing Dominic. I also used the advantage that he had all the pressure on him."
Gerald Melzer reached his first ATP World Tour quarter-final of the year with a 6-3, 6-1 win over Spain's Daniel Gimeno-Traver.
The Melzer brothers have played once before, with Jurgen winning last year in the opening round of qualifying for Wimbledon.