Cirstea, who had won all four previous matches against Voegele, held serve only once. Her service game was woeful, while Voegele got away with making only a third of her first serves, and dropping serve only once.
The Romanian, who enjoyed a career-best year in 2013, has reached only two quarterfinals this year, and none since February. Because of a right shoulder injury, this was Cirstea's second tournament in two months.
Voegele will face Shahar Peer of Israel on Friday, hoping to pass the quarterfinals for the first time this year.