Ratchanok Intanon
In February 2019, Yamaguchi played in the German Open. In the women's singles final, she defeated the No. 3 seed and World Championship champion Ratchanok Intanon 2–1 to become the Super 300 women's singles champion. In March, she played in the 2019 All England Open and lost to the tournament's top seed Tai Tzu-ying in the women's singles semifinal 1–2. In April, she played in the Malaysia Open, finishing as the runner-up after losing to the top seed of the tournament, Tai Tzu-ying, 0–2. She played in the Asian Championships. In the semifinal, she defeated Chen Yufei, the top seed in China, making it through to the final where she defeated He Bingjiao, winning the first Asian Championship title for the Japanese team. In the same month, she played in the New Zealand Open, losing in the semifinal to the tournament's No. 6 seed Li Xuerui 1–2. In May, she represented Japan at the 2019 Sudirman Cup held in Nanning, China, helping the Japanese team become the runner-up in the mixed group.