World No.1 Caroline Wozniacki's Australian Open preparations hit a setback when she was sent crashing out of the Sydney International by Dominika Cibulkova.

19176 - Wozniacki stumbles to opening defeat

The top seed had not lost a set to Cibulkova in their last five meetings, but she soon found herself on the back foot, after the Slovak snatched an early break en route to a 6-3 6-3 victory.

Cibulkova found herself a break down in the second set, but won four straight games to wrap up an impressive victory and seal her place in the quarter-finals.

"At the start of the new season you need to get into match play. I didn't feel I was playing great tennis out there," Wozniacki said.

"Now I just need to get some training matches, head to Melbourne and get ready for the Australian Open. Last year I had the same start, so hopefully slow start, good finish."

Second seed Vera Zvonareva was also upended in her opening match against Flavia Pennetta.

Kim Clijsters is the last of the top seeds standing after fourth seed Sam Stosur was beaten by a resurgent Svetlana Kuznetsova.