World No 2 Iga Swiatek won 6-2, 6-2 against Croatian qualifier Jana Fett in the last 16 of the WTA event in Stuttgart on Wednesday.
Swiatek broke her 153rd-ranked opponent twice in the first set and three times in the second.
The victory marked a return to winning ways for five-time Grand Slam winner Swiatek as she bounced back from a surprise defeat to Alexandra Eala in the quarterfinals of the Miami Open in March.
"You need a lot of inner energy and a lot of motivation to get out every week and play as good as you can," Swiatek said.
"It's not only about technique and tennis. It's about mentality and the feelings you have inside."
The Pole has twice won in Stuttgart, in 2022 and 2023. A third Stuttgart crown would be Swiatek's first tournament victory in 2025.
Rising star Mirra Andreeva booked her spot in the round of 16 earlier on Wednesday as she progressed when her elder sister Erika retired while trailing 6-2, 1-0.
The 17-year-old Russian will take on Ekaterina Alexandrova for a spot in the quarters.
World No 1 Aryna Sabalenka is already assured of a spot in the last eight despite not yet taking to the court in Stuttgart.
Sabalenka received a first-round bye before her scheduled last-16 opponent Anastasia Potapova announced her retirement from the tournament due to injury on social media.
The Belarusian has never won the title in Stuttgart, but finished runner-up three times between 2021 and 2023 – twice losing the final to Swiatek.
American world No 3 Jessica Pegula and countrywoman Coco Gauff, ranked fourth in the world, will be in action in the last 16 on Thursday.