Simona Halep was stunned in the second round of the Stuttgart Open as German qualifier Laura Siegemund pulled off a stunning straight-sets success over the world number six.
Halep's inconsistent 2016 has featured a first-round defeat at the Australian Open, as well as losses in her opening outing at two previous tournaments before this similar slip-up.
An ankle injury may have been hampering Halep, but Siegemund took advantage in stunning fashion, thrashing the Romanian 6-1 6-2.
"I think it'd be presumptuous to say [the match] was perfect, because there is always something to improve," Siegemund said. "But that was very good and of course I'm mega happy. I have not expected that it would go so smoothly.
"I just played point by point. Sure, it's definitely the biggest win of my career, but now after the match, I can only say, I have made few mistakes.
"I started very aggressively. That was the plan - attack her serve, tear open the field early. I know I've always done that well and it worked very well this time too."
Siegemund's reward is a quarter-final against Roberta Vinci, after the sixth seed saw off Julia Goerges 6-3 6-4.
Top seed Agnieszka Radwanska recovered from a slow start to beat Andrea Petkovic 1-6 6-1 6-2.
Playing her first match on clay of 2016, Radwanska was tested early on by Petkovic, but the German slipped nastily in the second set, to damage her chances.
"I slipped on the line and fell on my tailbone," Petkovic explained. "My ankle is okay, but my back is not so good. I just couldn't move anymore."
Karolina Pliskova awaits for Radwanska after beating Ana Ivanovic 6-4 6-2.
Fifth seed Petra Kvitova was another three-set winner, against Monica Niculescu, but Carla Suarez Navarro (7) was a more comfortable victor over Anna-Lena Friedsam.