Gauff is the youngest woman to reach the semifinals at a WTA event since Nicole Vaidisova won in Tashkent in 2004.
Gauff had thought she could be concerned in Luxembourg WTA qualifying this weekend, however finds herself with extra urgent duties in Linz as a substitute.
The current world number 110 is set to climb to at least 80th in the rankings next week.
Gauff needed a lucky loser spot to take her place in the main draw after failing to qualify but could claim the title on Sunday following a 6-4 6-4 defeat of Petkovic.More news: Opposition fans react to Klopp winning Manager of the Month award
"Last year around this time I was playing a juniors Grade 1 [tournament], and I lost in the quarterfinals there, in Mexico", Gauff added.
Injury-hit Maria Sharapova withdrew from next week's Luxembourg WTA tournament on Thursday, opening the door instead for American teenager Coco Gauff, widely seen as one of the veteran Russian star's future successors as a Grand Slam champion.
"I didn't have any nerves until the match points came along. I've really gotten a lot of support from the crowd", she told a sellout audience in this upper Austrian regional capital.
'I was fighting for every point, even at break points down, I just tried to get the serve in and make her play a point'.
"I'm not the most confident with it but it worked well today", she said.More news: 1 dead, 3 missing after hotel under construction collapses in New Orleans
"I'm thinking of the homework I have to do tonight".
The youngest-ever victor of a WTA Tour title was two-time US Open champion Tracy Austin, aged simply 14, at Portland in 1977.
Now ranked 110th, Gauff showed real determination to save nine out of the 10 break points she faced, before she sealed victory on her second match point.
The American fought by within the ultimate sport to serve out victory, staving off three break factors and coming by on her second match level.More news: Fed Plans More Treasury Purchase To Control Lending Rates