Petkovic and Jankovic both got wild-card invitations into the clay-court tournament. Petkovic was once in the top 10 but the German missed months of action through various injuries and has dropped to 103rd. Jankovic is a former No. 1 who is No. 16.
In Saturday's final, Petkovic will play either Simona Halep or Lucie Safarova.
Petkovic dropped her serve early in the match but recovered to take the first set.
Serving for the match, Petkovic fell into a 0-40 hole, only to see the Serb produce error after error and end the match with a backhand that sailed wide.
Results from the fifth day of the WTA Nuremburg Open (x denotes seeding):
Andrea Petkovic (GER) bt Jelena Jankovic (SRB x1) 6-4, 6-3
Simona Halep (ROM x7) bt Lucie Safarova (CZE x5) 6-3, 0-6, 6-2