Naomi Osaka's title defence at her home Pan Pacific Open has ended with the former world No.1 withdrawing ahead of her second-round match against Beatriz Haddad Maia due to illness.
"I am really sorry that I am not able to compete today," said the four-time grand slam champion, who won the tournament when it was last held in 2019 in her hometown of Osaka.
"This has and always will be a special tournament for me and I wish I could have stepped on court today, but my body won't let me."
It is the latest setback for the 24-year-old, who sat out the entire grasscourt season with an Achilles injury and also retired from her first-round match in Toronto with a back problem in the leadup to the US Open.
Before the Tokyo tournament, Osaka said she was fit and healthy.
She had reached the second round when Australia's Daria Saville was forced to withdraw with a torn ACL suffered in just the second game of their opening-round match on Monday.
Fifth-seeded Beatriz Haddad Maia of Brazil benefited by a walkover from Osaka's withdrawal and has a quarter-final against fourth-seeded Veronika Kudermetova, who dominated Mexican qualifier Fernanda Contreras Gomez 6-0 6-1.
In other second-round matches, Petra Martic upset sixth-seeded Karolina Pliskova 6-3 6-4, and in a match of unseeded players, Liudmila Samsonova squeezed past China's Xinyu Wang 7-6 (7-5) 6-3.
In Friday's other quarterfinals, Martic will play China's Zhang Shuai, Samsonova faces third-seeded Garbine Muguruza, and America's Claire Liu meets China's Qinwen Zheng.