In March 2015, Yamaguchi reached the women's singles semifinals in the Swiss Open. In May, she represented the Japanese team in the Sudirman Cup in her second singles appearance. Although Yamaguchi was not scheduled to play, in the final against China, the defending champions, the coach arranged for her to play since she had defeated both of China's top players. The Chinese team took a 2–0 lead in the women's finals. Yamaguchi's opponent was the London Olympics gold medalist Li Xuerui. Yamaguchi defeated her 23–21, 21–14, and the Japanese team was the runner-up. This marked the first time Japan had earned a victory in the Sudirman Cup Championship.