Stephens, who is starting to get her groove back, recently started a partnership with coach Sven Groenefeld. In April, the 26-year-old American competed in her first quarterfinal match of the season at the Volvo Open in Charleston, losing to countrywoman and eventual winner Madison Keys.
The 2018 French Open finalist will next play Petra Martic for a place in the final four. The Croatian advanced after compatriot Donna Vekic retired in the third set with an unspecified injury. Martic had received a walkover in the second round after Angelique Kerber withdrew with an ankle injury on Tuesday.