Petkovic, a former top-10 player who has dropped to 29th, met Muguruza with a 2-0 record against the Spaniard, but both wins were before 2014; Muguruza has risen to a career-high ranking of No. 5 since.
Zheng knocked out Australian Open champion Angelique Kerber in the second round but struggled to match the same high standard.
Radwanska's reward was a semifinal with eighth-seeded Carla Suarez Navarro, who put out Elena Vesnina of Russia 6-3, 7-6 (3). The result ensured Suarez Navarro will return to the top-10 rankings next week.
Petkovic, a winner of six WTA titles, broke Muguruza to start the match and quickly wrapped up the first set 6-1. They were 5-5 in the second when Muguruza earned her second break and took the set.
In the deciding set, Petkovic established her supremacy by breaking in the third and fifth games. She hit just 16 unforced errors compared to Muguruza's 38.
"It was so tough; Garbine is such a talented and dangerous player who put me under pressure," Petkovic said. "I was a little lucky in the end.
"I've been playing really well the past few weeks. Everything is coming together, and I'm getting my match focus again."