Naomi Osaka’s title defence at her home Pan Pacific Open event came to a premature end as she withdrew from her second-round match against Beatriz Haddad Maia.
The former world number one, who won the Tokyo tournament when it was last held in 2019, was unable to compete in Thursday’s match due to illness.
“I am really sorry that I am not able to compete today,” Osaka said. “It’s an honour to be able to play at the Pan Pacific Open in front of the amazing fans here in Japan.
“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.”
Haddad Maia advanced on a walkover and will next face fourth seed Veronika Kudermetova, who defeated Mexican qualifier Fernanda Contreras Gomez 6-0 6-1 to reach the quarter-finals.
Liudmila Samsonova also progressed to the last eight, with a 7-6 (7-5) 6-3 win over Wang Xinyu continuing her good form.
However, sixth seed Karolina Pliskova exited the competition on Thursday at the hands of Petra Martic, who prevailed 6-3 6-4 and will face Zhang Shuai next.
Ostapenko remains on a possible semi-final collision course with Emma Raducanu, who came through 6-4 7-5 against Yanina Wickmayer, while Magda Linette saved two match points to beat Kristina Mladenovic 4-6 7-6 (7-5) 6-2.