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Osaka, who announced last week she had split from coach Sascha Bajin, was stunned by Kristina Mladenovic in her first match as world number one, losing in straight sets in just over an hour after a shoddy display.

But the majority of the other seeds in the draw progressed, including Svitolina and Simona Halep, who breezed past Eugenie Bouchard.

Petra Kvitova and Angelique Kerber both battled back from early deficits, while Karolina Pliskova beat Dominika Cibulkova, with Garbine Muguruza easing to victory over Zheng Saisai. Aryna Sabalenka thrashed Ivana Jorovic.

Some seeds did join Osaka in exiting, though, with Kiki Bertens departing after defeat in a third-set tie-break to Viktoria Kuzmova, while Caroline Garcia went down to Jennifer Brady in straight sets.