Santangelo, the top seed in this week's event, dropped the first set and needed 12 games before claiming the second in a 3-6, 7-5, 6-2 victory over Akgul Amanmuradova.
No. 2 seeded Sania Mirza, a local favorite and the 2005 tournament champion, posted an identical 3-6, 7-5, 6-2 win in her match against Tatiana Poutchek.
Santangelo advanced to Friday's quarterfinals against Yurika Sema while Mirza next goes against Yaroslava Shvedova. The other quarterfinals matches send Jelena Kostanic Tosic against Olga Savchuk and Melinda Czink against Tzipora Obziler.
Costly malfunction causes beer flood at Boston-area brewery
Video of Victoria’s Secret models trying to 'twerk' hits Instagram