She reached the semi-finals in the women's singles event and won the bronze medal, losing to Nozomi Okuhara with a score of 19–21 and 9–21.
[11] One year after losing the final at the world juniors competition, Yamaguchi won the title by beating her teammate Aya Ohori.
At the 2nd Asian Youth Games held in Nanjing, China in August 2013, where she represented Fukushima Prefectural Tomioka High School, she won the gold medal in mixed doubles.
[14] In October, she represented Japan at the East Asian Games held in Tianjin, China, where she finished third in the women's singles event, losing in three sets to Wang Shixian.
[17] At the Malaysian Open in January, Yamaguchi defeated reigning world champion Ratchanok Intanon in the first round in three games.
[22] On 16 August, Yamaguchi served as the flag-bearer for the Japanese team at the opening ceremony of the 2014 Summer Youth Olympics in Nanjing, China.
[24] At the China Open in November, Yamaguchi defeated top ranked players in two straight games, Wang Shixian, Sun Yu, Bae Yeon-ju and reached the final.
[26] In the second round of the Indonesian Open in June, Yamaguchi beat the reigning Olympic champion Li Xuerui.
[29] In August 2016, Yamaguchi represented her country at the 2016 Summer Olympics held in Rio de Janeiro, Brazil.
On her way to victory, Yamaguchi defeated her teammate Okuhara in the quarter-finals and two-time World Champion and Olympic gold medalist Carolina Marín in a close three-set match in the semi-finals.
[36] In April, she participated in the Asian Championships held in Wuhan, China, losing to Tai Tzu-ying in the final.
[40] In October, she played in the French Open and lost the final to the top seed, Tai Tzu-ying in a one-sided match.
[42] In December, she played in the Dubai World Superseries Finals, defeating Sayaka Sato and He Bingjiao and losing to India's P. V. Sindhu in the group stage.
In the top four, facing the Olympic runner-up P. V. Sindhu, she lost both games, finishing in third place in the World Championships women's singles.
[50] In August, she represented Japan in the Asian Games held in Jakarta, Indonesia, and helped the Japanese women's team win the gold medal.
[52] In a repeat clash of previous edition's French Open final, Yamaguchi beat the top seeded Tai Tzu-ying in three games, 22–20, 17–21, 21–13.
[58] That same month, she won a match against Taiwanese competitor Tai Tzu-ying, the highest-ranked player in the world, placing her in the finals of her first Indonesia Open.
[59] In August, Yamaguchi suffered AN injury and was defeated in the second round of the 2019 BWF World Championships held in Basel, Switzerland, by 20-year-old Yeo Jia Min of Singapore 0–2.
[citation needed] In 2020, Yamaguchi claimed her first title victory of the year at the Thailand Masters, winning against An Se-young in two consecutive games.
[70] She became the fourth Japanese women's singles player to win the All England Championship after Hiroe Yuki, Etsuko Takenaka and Nozomi Okuhara.
[75] At the BWF World Tour Final held in Bangkok, Thailand in December, she won the championship with a 2–0 victory over Tai Tzu-ying.
[76] Chinese player Sun Yu has commented on Yamaguchi's style, pointing out that while small, she is diligent and runs swiftly, and she is capable of picking up the kinds of difficult shots that others are typically unable to save.
Another Chinese player, Wang Shixian, agrees that Yamaguchi is quick and nimble, and for challengers to be successful, they need to be capable of matching her speed.
[77] In 2019, Li Yongbo, head coach of the Chinese team, commented on the Japanese player: She plays with patience and quality.
Girls' singles Mixed doubles Yamaguchi's record against year-end finals finalists, world championships semi-finalists, and Olympic quarter-finalists.