Afterwards, Osaka, who turns 21 on October 16, said it was more a case of fatigue than anything more serious, but suggested that she may not make the WTA Hong Kong Open, which starts on Monday.
"Unfortunately, I have been playing through pain in my back this whole week," the world number six said in a subsequent statement released by organisers.
"The doctors have advised me that I need to rest to ensure the injury is not further aggravated and become more serious." One player who does appear set to play in Hong Kong is China's Wang Qiang, who is in the form of her life.
"I'll try my best because I will definitely face the same situation in the future," the 28th-ranked Wang said, asked if she would compete in Hong Kong.
"I cannot give up easily.
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