The 24-year-old Spanish defending champion, ranked seventh in the world, lost 2-6 6-4 6-4 to her Ukrainian opponent, the world number 45.

It was Muguruza's first match since her defeat in the Wimbledon second round, where she was also defending her title.

"It was a very ugly match, but I'm happy that I fought, even if the level wasn't very high," said Muguruza.

"At the end she managed to come back in the third set and play better and found a way to win the match."

World number one Simona Halep was still on court when rain halted play.

The top-seeded Romanian stood 4-6 6-3 3-3 against Australian qualifier Ajla Tomljanovic when storms rolled in to bring the night session to an end on a day that had already seen one three-hour rain delay.

Earlier on Wednesday, American world number three and 2017 US Open champion Sloane Stephens set up a third-round meeting with Elise Mertens by beating Germany's Tatjana Maria 6-3 6-2.

Belgium's Mertens, the world number 14, beat Rebecca Patterson of Sweden 3-6 6-2 7-6 (7-1).

Wimbledon champion Angelique Kerber claimed her 40th win of the season, but the German fourth seed had to recover from a set down to overcome Russia's Anastasia Pavlyuchenkova 4-6 7-5 6-4.

Second seed Caroline Wozniacki retired with a left knee injury after surrendering the first set 6-4 to Kiki Bertens.