Former champion Maria Sharapova has lost in the fourth round of the Australian Open to Andrea Petkovic.

The 30th-seeded Petkovic of Germany was rarely challenged in beating the 2008 Australian Open champion 6-2, 6-3 on Sunday. Petkovic benefited from an easy third round — she only needed to play seven points before seven-time major winner Venus Williams retired on Thursday night with an injured hip muscle.

She will now take on China's Li Na in the quarterfinals.