BASTAD, SWEDEN - World No. 1 Caroline Wozniacki retired from her second-round match Wednesday at the $220,000 Swedish Open, citing a right shoulder injury.

The Danish star was leading Sweden's own Sofia Arvidsson 6-2, 0-1 when she quit on the red clay at Bastad Tennis Stadium. Wozniacki was the Bastad runner-up two years ago.

The 20-year-old Wozniacki was fresh off her disappointing fourth-round finish at Wimbledon last week. The busy star was playing in her 15th tour event of the year and owns five titles on the WTA in 2011.

Meanwhile, second-seeded Flavia Pennetta is still standing after whipping German Angelique Kerber 6-2, 6-3, while another seed exited the draw when Swedish favorite Johanna Larsson overcame No. 5 Russian Vera Dushevina 6-7 (4-7), 6-2, 6-2.

Three other seeded women joined the Italian Pennetta in the quarterfinals, as No. 4 Lourdes Dominguez Lino handled fellow Spaniard Arantxa Parra Santonja 7-5, 6-1, No. 7 Czech Barbora Zahlavova Strycova outlasted Belarusian Anastasiya Yakimova 7-5, 4-6, 6-3, and No. 8 Slovenian Polona Hercog came back to best defending Bastad titlist Aravane Rezai 3-6, 6-4, 7-6 (7-4). The French Rezai beat Argentine Gisela Dulko in last year's finale here.

In other second-round action, Russian Vesna Dolonts pasted Romanian Sorana Cirstea 6-3, 6-3 and Maria Jose Martinez Sanchez subdued fellow Spaniard Nuria Llagostera Vives 7-5, 6-1.

Thursday's quarterfinals will pit Pennetta against Zahlavova Strycova, Dominguez Lino versus Larsson, Hercog against Dolonts and Arvidsson versus Martinez Sanchez.

The 2011 Swedish Open champion will pocket $37,000.