World number one Iga Swiatek reached the quarter-finals of the WTA Stuttgart event with a straight-sets win over Zheng Qinwen on Thursday on her return from injury.

The Pole won 6-1, 6-4 as a dominant first set put her on course for victory in just under 90 minutes at the clay-court tournament.

"I felt like I needed to be 100 percent focused from the beginning to the end, to think about technique and tactics," Swiatek said. "I'm pretty happy that I'm not rusty and I could perform good tennis, even though I had a break."

After Zheng broke to lock the second set at 4-4, the two-time French Open winner broke back immediately and then served out the match.

Swiatek withdrew from the Miami Open and Billie Jean King Cup qualifiers with a rib injury, but told reporters on Wednesday: "I'm not injured anymore."

Earlier, Coco Gauff's French Open warm-up stalled as last year's Roland Garros runner-up fell in the last 16 to Anastasia Potapova.

The Russian impressed with her returning game to dominate the American, winning 6-2, 6-3.

World number six Gauff, who made it past Potapova's compatriot Veronika Kudermetova in three sets despite an error-ridden second-round performance on Wednesday, converted none of her six break points against Potapova.

Potapova, ranked 24th, is competing in Stuttgart for the first time with the 22-year-old advancing to her fourth quarterfinal of the year.

Earlier on Thursday, Czech former world number one Karolina Pliskova fought past Croatian Donna Vekic 6-2, 6-7 (5/7), 7-6 (7/5).

Pliskova, who won at Stuttgart in 2018, dominated the first set but lost the second in a tie-break. She clinched victory in the third in another tie-break.