Maria Sharapova expressed her frustration at being able to "consistently" find her best level, as she suffered another disappointing Grand Slam result in 2018.
"I definitely expected more from the Slams this year", said the 31-year-old. The belief matters most when it's internal and when you have a passion for something.
"Madison is from the United States so I guess all the crowd will be for her", the Spaniard added.
Vondrousova accused Tsurenko of putting on an act, but the Ukrainian insisted her struggles were real, saying: "I don't know what to say. But today was one of the toughest matches in my life". Sharapova tried to fight back, missed another game point in the final game but failed a couple of routine backhands while Suárez Navarro converted on her first match point with the same smart, outstanding play that brought her there, and sealed the win striking her 20th victor of the match.
The 14th-seeded American, looking to go one better than her runner-up finish at Flushing Meadows past year, relied on both her unsafe serve and powerful groundstrokes to deny Cibulkova any chance of an upset win.
Navarro will play to make her maiden Grand Slam semifinal against 2017 finalist Madison Keys.
But she ran into an inspired Suarez Navarro who marked her 30th birthday with a second win in five meetings against the Russian.
Since returning from her drugs ban a year ago, Sharapova has made just one quarter-final in five grand slams.
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"If I didn't have the belief to keep doing this and to keep having the motivation and the grind of doing this every day in order to get myself in these positions, I don't think I would be here", said Sharapova. I think I've done plenty in my career, established a lot for myself personally, professionally.
'Yeah, the belief is not something that I'm eager to show everybody else.
Sabalenka, who saved three break points in the final set, flung her racket in dismay after committing a double-fault on match point, handing victory to her 19th-ranked opponent.
The No. 20 seed had to work much harder than she had so far in the tournament, where she had won 22 consecutive games leading into Sunday's match.
The other quarterfinal in the bottom part of the draw will see Japan's Naomi Osaka face Lesia Tsurenko of Ukraine.
In the two-hour 32-minute match, which featured 13 breaks of serve, Tsurenko committed 57 unforced errors while 19-year-old Vondrousova hit a huge 73. But she has never won a Grand Slam quarterfinal, losing her previous six. He also blew a set point in the third. "My win against Garbiñe Muguruza in Cincinnati and some victories there gave me the confidence that I can play well".
Keys cruised to a 6-1 6-3 victory over Slovakia's Dominika Cibulkova earlier on Monday, relying on her serve and powerful groundstrokes to overpower the Slovakian.
"It's no secret that Serena is my favourite", she said.