Thus, Cori has become the youngest WTA semi-finalist since Nicole Vaidisova in Tashkent 2004 and the youngest American since Jennifer Capriati since the US Open 1991, achieving the feat before Venus and Serena Williams.
Petkovic beat Slovakian Viktoria Kuzmova 6-4 6-1, the German reaching her first semi-final of 2019.
The Latvian has endured a miserable season and has dropped outside the top 70 but, working with former Wimbledon victor Marion Bartoli, this week has seen a resurgence.
"This kind of proves I'm able to hang in on this level and I'm really proud of myself today".
"This is actually the first time in my life, including juniors, that I got into something as a lucky loser, and now I'm in the final", said Gauff. "I guess Linz is my special place, giving me a lot of luck and a lot of happiness here".More news: Rams' Gurley misses practice with thigh injury, listed as day-to-day
More news: Bella Hadid 'still friends' with The Weeknd
More news: Former Ukraine envoy testifies Trump pushed to oust her
Gauff showed plenty of variety in her game as she beat the 75th-ranked Petkovic, relying frequently on well-executed drop shots.
"I didn't have any nerves until the match points came along".
After taking the opening set in a tiebreak, Gauff needed only one break of serve to wrap up the match in 97 minutes for her first career win over a top-10 player.
Gauff, who fell in the second round of Upper Austria Ladies Linz qualifying, gained entry into the main draw due to a withdrawal and has made a resounding run this week.
Gauff, just 15, burst onto the scene at Wimbledon as she reached the fourth round, before impressing again at the US Open, her home grand slam, by winning two matches.