Oct. 25—MINNEAPOLIS — Never underestimate the heart of a champion. That adage befit Rochester Mayo’s Claire Loftus perfectly Friday afternoon in the Class 2A tennis tournament singles championship at the University of Minnesota’s Baseline Tennis Center. The Mayo senior went from looking exhausted and beaten, to a champion, beating a game Ava Nelson of Elk River 5-7, 7-5, 2-0, the final set …