Eating out, poor hand hygiene and erratic weather may be among causes, say medics
The fifth-seeded Zhang was trailing 4-1 when she retired against Wang in hot and humid conditions.
"I'm not feeling well. I feel so tired since the Australian Open," Zhang said.
"I still felt OK in the first round (on Tuesday). But on Wednesday my body really felt bad. I'm sorry that I have to retire."
Wang advanced to a quarterfinal on Friday against Magda Linette of Poland, who overcame Monica Niculescu of Romania 4-6, 6-3, 6-2 in 2 ½ hours. Niculescu took a medical timeout at the start of the second set. She looked better in the third set than the second, but Wang rode her momentum.
Also, Viktorija Golubic of Switzerland defeated qualifier Duan Yingying of China 7-6 (4), 7-5.
Eating out, poor hand hygiene and erratic weather may be among causes, say medics
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