In 2005, Ziegler won the 800-meter freestyle (8:25.31) and 1,500-meter freestyle (16:00.41) at the World Championships in Montreal. The latter time made her the third-fastest woman (and second-fastest American) in the history of the event, following only world-record holder Janet Evans's 15:52.10 and German Hannah Stockbauer's 16:00.18. She qualified for Worlds after winning the 800-meter freestyle at World Trials in Indianapolis with a time of 8:34.83. She failed to qualify in the 400-meter freestyle, finishing third in a time of 4:12.09.