World number one Iga Swiatek marked her return from injury with a straight-set win over China's Zheng Qinwen at the Porsche Tennis Grand Prix.

The 21-year-old had not played since last month's Indian Wells semi-finals, withdrawing from the Miami Open and Billie Jean King Cup qualifiers with a rib injury.

"I'm really happy that I'm not rusty," Swiatek said after the match.

"It was difficult, especially because I didn't play since Indian Wells, so I felt like I really needed to be 100% focused."

The three-time major winner will face Karolina Pliskova in the quarter-finals, after the Czech former world number one beat Croatia's Donna Vekic 6-2 6-7 (5-7) 7-6 (7-5).

Playing in her first clay-court match of the season, Swiatek eased through the opening set and did not face a break point in her first seven service games.

Zheng tested Swiatek in the second set, winning back-to-back games before the defending French Open champion broke again to lead 5-4 and held serve to wrap up the match.

Elsewhere on Thursday, American fifth seed Coco Guaff lost 6-2 6-3 to Russia's Anastasia Potapova, who will play Caroline Garcia in the next round after the fourth seed beat Germany's Tatjana Maria 7-6 (7-5) 6-4.

Meanwhile, Beatriz Haddad Maia advanced via retirement for a second straight match as Wimbledon champion Elena Rybakina was forced to withdraw due to a hip injury with her Brazilian opponent 6-1 3-1 up.

Spain's Paula Badosa won 6-1 6-2 against compatriot Cristina Bucsa, setting up a last-eight tie against world number two Aryna Sabalenka.