Tai Tzu-ying
In March, Yamaguchi played in the Super 300 German Open tournament. She won the title after defeating the No. 4 seed, China's Chen Yufei, in the final. She also played in the All England Open in March, losing to Tai Tzu-ying in the final. In April, Yamaguchi became the first Japanese person to become the top-ranked singles player in the world, for which she was awarded the "Honorary Citizen Award" by the mayor of Katsuyama, Japan, on 1 May 2018. In May, she was selected as the main force of the Uber Cup women's singles, helping the Japanese team regain the women's team championship after 48 years.