Petkovic is coming back from a series of injuries and got a wild card into the tournament, like Jankovic. Petkovic came from a set down to beat eighth-seeded Annika Beck 4-6, 6-3, 7-5.
In the other semi-final Friday, fifth-seeded Lucie Safarova will play No. 7 Simona Halep.
Safarova needed three sets to edge Polona Hercog 7-6 (5), 3-6, 6-4, while Halep defeated Galina Voskoboeva 6-4, 6-1.
Results from the fourth day of the WTA Nuremburg Open (x denotes seeding):
Jelena Jankovic (SRB x1) bt Lourdes Dominguez (ESP x6) 6-4, 6-0
Andrea Petkovic (GER) bt Annika Beck (GER x8) 4-6, 6-3, 7-5
Simona Halep (ROM x7) bt Galina Voskoboeva (KAZ) 6-4, 6-1
Lucie Safarova (CZE x5) bt Polona Hercog (SLO) 7-6 (7/5), 3-6, 6-4