Yahoo Inc. Chairman Terry Semel stepped down as chief executive in a surprise move Monday, ending his increasingly ineffectual pursuit of online search leader Google Inc., a losing battle that had demoralized Yahoo's shareholders and employees. The Sunnyvale-based company appointed co-founder Jerry Yang as its new CEO and named Susan Decker as its president. Decker, who had been touted as Semel's heir apparent, was recently promoted from Yahoo's chief financial officer to oversee the company's advertising operations. Semel, 64, will remain chairman in a non-executive role after spending the past six years running the company. "I saw myself as more of a coach than a player going forward," Semel told analysts and media during a Monday conference call.