World number one Serena Williams has eased into the semi-finals of the Australian Open. The top seeded American beat fifth seed Maria Sharapova in straight sets, winning 6-4, 6-1. Sharapova has now lost 18 times in a row against Williams. The victory sets up a last-four meeting with fourth seed Agnieszka Radwanska, who beat Carla Suarez Navarro 6-1, 6-3.