The match lasted 1 hour and 26 minutes, extending her unbeaten streak at Stuttgart to 5.
“I felt like I needed to be 100% focused from the beginning to the end, to think about technique and tactics,” Swiatek said afterward.
“I’m pretty happy that I’m not rusty and I could perform good tennis, even though I had a break.”
The Stuttgart Open is the first time that Swiatek is playing again after 45 days.
She had taken some time off following a rib injury that forced her to miss the Miami Open and the Billie Jean King Cup qualifier. Swiatek showed no signs of the injury during her match with Zheng.
She broke Zheng twice in each set. The match this time only lasted two sets this time but in their previous meetings, Zheng had pushed Swiatek to a third set.
“She’s really talented, and everybody can see that,” Swiatek said.
“I needed to adjust, and I’m just happy that I was focused on myself, on what I wanted to do … and I feel pretty comfortable. So I’m happy that I could show my good tennis.”
She next faces Pliskova in the final eight. Pliskova defeated Donna Vekic in her match that lasted 2 hours and 20 minutes and needed a third-set tie-break.
Swiatek and Pliskova have only played each other once in the past and it overwhelming went in Swiatek’s way, scoring 6-0 6-0 and losing only 13 points.