Naomi Osaka's title defense at her home Pan Pacific Open ended Thursday when the former world No. 1 withdrew ahead of her second-round match against Beatriz Haddad Maia due to abdominal pain.
"I am really sorry that I am not able to compete today," Osaka said in a statement. "It's an honor to be able to play at the Toray 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, but my body won't let me. I will see you next year."
It is the latest setback for the 24-year-old, who sat out the entire grass-court season with an Achilles injury and retired from her first-round match in Toronto with a back problem in the lead-up to the US Open. The four-time Grand Slam winner had won the Pan Pacific Open when it was last held in 2019 in her hometown of Osaka.
In the quarterfinals, Brazil's Haddad Maia will face No. 4 seed 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 (5), 6-3.
In Friday's quarterfinals, Martic will play China's Zhang Shuai, Samsonova faces third-seeded Garbine Muguruza, and America's Claire Liu meets China's Qinwen Zheng.
Martic kept her nerve as 2018 champion Pliskova lost hers. The former world No. 1 was not at her best, while Martic played steadily throughout, breaking twice in each set, including the final games of each set. Pliskova handed the win to her fellow tall baseliner with her fifth double fault.
"Karolina is a great player and she's beaten me many times in the past, so I expected a tough match," Martic said. "So I was just happy to win it."
Martic won in the nick of time as it started to rain right at the end of the second set. A third set would have been under the roof of Ariake Coliseum.
"I was hoping we would play outdoors because it's an outdoor tournament, but at match point it started to rain really hard, so I was really happy to win that point," Martic said.
Samsonova outslugged Wang as both players stayed aggressive throughout. Samsonova hit three aces to wrap up the first-set tiebreak. In the second set, her high-pressure play broke Wang in the eighth game and she rounded off the match with a 13th ace.
"She was playing amazing," Samsonova said. "She's a great player already, but my serves were enough to maintain my game."
Contreras Gomez had no answer for Kudermetova's clinical efficiency, as the good parts of her game that saw her defeat 2020 Australian Open champion Sofia Kenin in the first round deserted her. Kudermetova didn't put a foot wrong but should face far sterner opposition from Haddad Maia in the quarterfinals.
The Associated Press and Reuters contributed to this report.