The neighbor - identified only as Sonia B - claims she suffered noise pollution between 2003 and 2007 due to the five-days-a-week, eight-hour practice sessions of Laia Martin, who lived below her in the northeastern town of Puigcerda.
Martin, 27, now a professional concert pianist, denies that she played at home that often, saying she took regular classes in other towns.
The El Pais newspaper said Sonia B. told the court Monday that she now hated pianos and eventually moved.
A Catalonia Justice Tribunal spokeswoman said the trial continued Tuesday and ends Nov. 15. She spoke on condition of anonymity because of tribunal regulations.