MELBOURNE (Reuters) - Agnieszka Radwanska continued her perfect start to the year, defeating former world number one Ana Ivanovic 6-2 6-4 to march into the quarter-finals of the Australian Open on Sunday.

The 23-year-old made just four unforced errors to 34 for her Serbian opponent and saved five break points in the second set before sealing victory with her seventh ace after 70 minutes.

Radwanska won the titles at both her warm-up tournaments in Auckland and Sydney and has not dropped a set in 13 straight victories this year.

The Pole will next face China's sixth seed Li Na, who she beat in the semi-finals at the Sydney International 10 days ago.

(Reporting by Ian Ransom/Nick Mulvenney; Editing by Patrick Johnston)