The 15-year-old American needed a lucky-loser spot to get into the main draw but has taken her opportunity superbly, and a 7-6 (1) 6-4 victory over world number eight Kiki Bertens gave her the biggest win by ranking of her fledgling career.
Gauff, who was already guaranteed to break into the top 100 on Monday, is the youngest woman to reach a WTA Tour semi-final since Nicole Vaidisova won Tashkent in October 2004.
"It was definitely crazy," Gauff told reporters. "Today was my first top-10 win, so I'm super proud of myself for that.
"It was a good match, Kiki played great, and obviously I couldn't ask for anything better so far in this tournament.
"I guess (Linz is) my little lucky place. I got in as a lucky loser and I'm in the semis, so I hope I can keep this going."
Bertens' defeat also means she still has work to do to secure a place at the WTA Finals.
In the last four, Gauff will take on Germany's Andrea Petkovic, who defeated Viktoria Kuzmova 6-4 6-1.
Eighth seed Ekaterina Alexandrova of Russia booked her place with a 6-2 6-4 win over France's Kristina Mladenovic. Latvia's Jelena Ostapenko overcame Kazakhstan's Elena Rybakina 7-5 6-1.
At the Tianjin Open, Veronika Kudermetova knocked out third seed Dayana Yastremska with a convincing 6-4 6-0 victory.
She is bidding to reach her first WTA Tour final, and to do so must get past a resurgent Heather Watson, who followed up her upset of Wang Qiang with a 7-5 6-7 (4) 7-6 (6) victory over Magda Linette that took more than three hours.
Ons Jabeur won two tie-breaks to see off sixth seed Yulia Putintseva and will face Swede Rebecca Peterson in the last four.
