In-form teenager Mirra Andreeva crashed out of the WTA event in Stuttgart on Thursday after going down in straight sets to fellow Russian Ekaterina Alexandrova in the last 16.
World number seven Andreeva, who already has two titles under her belt this season, lost 6-3, 6-2 against the 22nd-ranked Alexandrova in just over an hour.
The 17-year-old Andreeva had defeated her elder sister Erika in the previous round on Wednesday but Alexandrova quickly took control as she claimed her fourth win over a top-10 player this season.
In February, the 17-year-old Andreeva became the youngest winner of a WTA 1000 event in Dubai, beating Iga Swiatek and Elena Rybakina on her way to the crown.
She backed it up by winning at Indian Wells in March, coming from a set down to defeat world number one Aryna Sabalenka in the final.
Later on Thursday, American world number four Coco Gauff made short work of German Ella Seidel, winning 6-1, 6-1 in 57 minutes.
Gauff rebounded from her straight sets exit in the last 16 at the Miami Open, hitting two aces and breaking her opponent five times en route to a dominant victory.
Gauff will meet Italian fifth seed Jasmine Paolini, who beat German Jule Niemeier 6-1, 7-5 to reach the Stuttgart quarters for the second straight year on Thursday.
"I'm doing what I love, I think just be happy about it, try your best and enjoy," said Paolini.
"That's the key for me to play better. When I'm sad and complaining, I'm not that good."
Alexandrova will face third-ranked American Jessica Pegula, who beat Poland's Magdalena Frech 6-1, 6-1. Pegula won five from five break points and fought off one against her.
Sabalenka has already qualified for the quarter-finals after her opponent Anastasia Potapova pulled out of their last 16 clash with injury.
World number two Swiatek, a two-time winner in Stuttgart, has also reached the last eight.