Jankovic posted a 6-3, 6-2 victory over Mariana Duque Marino, who won this event three years ago.
Alexandra Cadantu will face Jankovic in the quarterfinals after rallying past Tereza Mrdeza 4-6, 6-3, 6-4.
Schiavone's match against Paula Ormaechea was suspended on Wednesday because of rain, and when they resumed Thursday it was Ormaechea who prevailed 2-6, 6-3, 7-6 (7-3).
In addition to the Jankovic-Cadantu match, Friday's quarterfinals will send Knapp against Arruabarrena-Vecino, Maria-Teresa Torro-Flor against Ormaechea and Mandy Minella against Teliana Pereira.
Ray Liotta sues skin care company over use of likeness
Millions of Getty images now available for free via embed tool