Committed mischief-maker Anastasija Sevastova concocted disorientating spins shredding Maria Sharapova, 5-7, 6-4, 6-2 to rally into her second straight US Open quarterfinal.
Sharapova ended the match with 51 errors to 42 winners as she pressed too hard in trying to break down Sevastova's brilliant defence.
The marquee match of the Round of 16 kicks off the night session on Arthur Ashe Stadium, as Muguruza looks to keep her charge for the No.1 ranking going.
Williams dominated that third set though, and took a win without issue.
As for Sharapova, the Russian wildcard came out of the gates firing against No.2 Simona Halep in her opening match, able to maintain an incredibly high level over the course of their memorable three-match encounter.
Sharapova, noting it wasn't her decision who played where, shook the shot off with aplomb.
Sharapova blasted a backhand down-the-line victor to break for a 3-1 edge.
"The first set was very close, it could have gone either way", said Sevastova.
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She clinched the set when Sharapova overcooked her return - and immediately left the court.
Sevastova, a quarter-finalist in NY also in 2016, said she was happy to see Sharapova playing again and that she felt no desire to prove a point on behalf of her peers. Navarro then held, and then broke Williams again to go up 3-0. But Sevastova held at love, broke again and took the match on a service victor.
The 30-year-old Sharapova, who received a wild-card invitation to the tournament, lost by a score of 5-7, 6-4, 6-2 in the prestigious tournament to the 16th-seeded Latvian. "I don't think I played as aggressive or was stepping in as much as I did in the first set, especially in the beginning of the match".
This was the biggest victory of Kvitova's return to competition after a knife attack by an intruder at her home in the Czech Republic in December. If you put me out in the parking lot of Queens in New York City, I'm happy to play there.
The Russian was bidding to make the second week of her first Grand Slam since returning from a drugs ban.
Sevastova will face unseeded American Sloane Stephens, who reached her first quarterfinal in NY by eliminating No. 30 Julia Goerges 6-3, 3-6, 6-1. Kvitova, a two-time Wimbledon victor, eliminated No. 3 seed Garbine Muguruza, 7-6 (7-3), 6-3. However, she couldn't finish off her opponent after winning the first set.
In a draw half assured of producing a first-time Slam finalist, he has a Tuesday quarter-final against either French 16th seed Lucas Pouille or Argentine 29th seed Diego Schwartzman.
Even as sister Serena is tending to her new-born baby girl, Venus Williams is vying for the title 20 years after she made it to the final on her US Open debut.
"As I said, I came in not playing a lot of matches. So I think that's probably where I kind of got a little fortunate there".