Yamaguchi competed as an eighteen-year-old and a member of the Japanese team in two freestyle relay events at the 2008 Summer Olympics in Beijing. Despite missing out the individual spot in the 200 m freestyle, she managed to place fourth at the Olympic trials in Tokyo (2:01.64) to earn an outright selection on the relay squad. Teaming with Haruka Ueda, Maki Mita, and Emi Takanabe in the 4 × 200 m freestyle relay final, Yamaguchi swam the second leg with a split of 1:58.51, but the Japanese team had to settle for seventh place in 7:57.56. Yamaguchi also participated in the 4 × 100 m freestyle relay, along with Ueda, Mita, and Asami Kitagawa, but missed out the top 8 final by 0.08 of a second in a prelims time of 3:39.25.