Kvitova had made the finals in two of her first three tournaments this year.
Williams, now ranked number ten in the world, has won a record 23 Grand Slam titles - one more than former record holder Steffi Graf.
Williams consulted a trainer before testing herself in the second set, but subsequently pulled the plug on her afternoon after falling behind 1-0. She emerged for the second set but it was short lived.
Following up on a strong showing Friday against Victoria Azarenka, the 23-time Grand Slam victor took a 3-0 lead over Muguruza, but lost the next six games and the first set.
"By the score it might have looked like I started well, but I was not feeling at all well physically". "It's extra motivation for me to play just a little bit better out there".
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She was found unresponsive, and a report from authorities in Texas say indications are it was suicide as she was found hanged. According to TMZ , she died by an apparent suicide , though a final cause of death is yet to be confirmed by a coroner.
Mona Barthel and Venus Williams meet in the fourth round of the 2019 tennis Indian Wells.
"Before the match I did not feel great, and then it just got worse with every second", Williams said afterward (via WTA Insider).
She is now ranked 10th in the world.
"It's really a weird feeling, because I don't feel like I won the match point and [heard], 'Well done, good match, '" she said.
"I never played her, so it's also you're out there learning", Williams said.
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She retired after losing the first game.
"It was hard for me to close out those sets", Halep told reporters, praising Kozlova.
Halep next faces 61st-ranked Czech Marketa Vondrousova, who followed up her win over 2018 finalist Daria Kasatkina with a 4-6, 6-3, 6-4 win over 22nd seed Jelena Ostapenko of Latvia.
The Romanian was one of six seeds to progress, with Svitolina joining her after a 7-5 6-4 win over Daria Gavrilova.
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Another passenger Sead Nikaj, told the network "nobody announced it or anything like that so we figured out something was wrong". Passengers who were on the flight described a range of injuries, including bloody noses and possible broken bones.