Kitzbuhel - Gerald Melzer came out on top in his Kitzbuhel ATP quarter-final duel with older brother Jurgen on Thursday with a 3-6, 6-3, 6-1 win.
Jurgen, 35, won their only previous meeting last year in the opening round of qualifying at Wimbledon.
But after dropping the first set on Thursday, 26-year-old Gerald, ranked at 107 in the world, rallied to win 10 of the last 11 games.
"It was very difficult for both of us. It's hard to face somebody you love that much because you don't want to hurt that person. Winning points just doesn't feel right," Gerald Melzer told ATP World Tour website.
"I might have been lucky with the rain break as the court became a bit slower. I was able to make returns that I didn't make in the first set and a half. Then we got more into rallies and that's when I felt way more comfortable."
Gerald Melzer will be playing in his second ATP World Tour semi-final where he will face Itralian fourth seed Paolo Lorenzi.
Lorenzi reached his third career semi-final by seeing off German eighth seed Jan-Lennard Struff 6-2, 6-7 (5/7), 7-5.
Results from the fourth day of the ATP event in Kitzbuhel on Thursday (x denotes seeding) - Quarter-finals:
Gerald Melzer (AUT) bt Jurgen Melzer (AUT) 3-6, 6-3, 6-1
Paolo Lorenzi (ITA) bt Jan-Lennard Struff (GER) 6-2, 6-7 (5/7), 7-5
Nikoloz Basilashvili (GEO) bt Adam Pavlsek (CZE) 5-7, 7-6 (7/2), 6-3
Dusan Lajovic (SRB x6) bt Karen Khachanov (RUS) 6-3, 6-2