Andrea Petkovic has withdrawn from the Australian Open with a stress fracture in her lower back that could keep her side-lined for two months.

World number one Caroline Wozniacki suffered a surprise fourth-round defeat by Germany’s Andrea Petkovic in the Sony Ericsson Open on Monday.

Andy Roddick fired 15 aces in a 6-4, 6-1 win over Ukranian Alexandr Dolgopolov to advance to the quarterfinals of the Brisbane International.