Number one seed Angelique Kerber crashed out of the Qatar Total Open in Doha when she was defeated 6-4 0-6 6-4 by Russian teenager Daria Kasatkina.
The 19-year-old, ranked 32 in the world, survived six break-point chances as she took the opening set.
Kerber, the current world number two, rallied to level the match after breezing through the second set in less than 30 minutes.
However, Kasatkina responded with two early breaks in the decider to lead 3-0 and then closed out victory to secure a quarter-final place against Monica Puig of Puerto Rico, which will be played later on Thursday.
Polish number four seed Agnieszka Radwanska is also out, having lost 7-5 6-3 to Denmark's Caroline Wozniacki.
Third seed Dominika Cibulkova, though, secured her place in the semi-finals after beating Samantha Stosur 7-5 6-4 for what was a first career victory over the Australian.
Second seed Karolina Pliskova will face China's Shuai Zhang after she beat Caroline Garcia of France in their second-round match 7-5 6-4, while Wozniacki is scheduled to play American Lauren Davis in the late match on Court One.