She set a Japanese record of 54.43 in the 100 m freestyle at the 2009 Japan National Sports Festival in Nagaoka, Niigata.

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.

[5][6] 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.

[7] In 2009, Yamaguchi earned a silver medal for the Japanese team in the 400 m freestyle relay at the Summer Universiade in Belgrade, Serbia with a final time of 3:42.60.