Third seed Jessica Pegula became the latest top-ranked player to bite the dust at the US Open on Monday as Carlos Alcaraz looked to keep his title defence on track.
A day after world number one and reigning women's champion Iga Swiatek was sent crashing out on Sunday, Pegula found herself heading for the exit as she slumped to defeat against American compatriot and close friend Madison Keys.
Pegula had gone into the US Open dreaming of a first ever Grand Slam title, buoyed by victory in last month's WTA 1000 Canadian Open in Montreal.
But the 29-year-old's US Open campaign came to an abrupt halt in front of a packed Arthur Ashe Stadium as 2017 US Open finalist Keys recorded a dominant 6-1, 6-3 win in just 61 minutes.
Pegula was left with no answer as Keys unleashed a stream of 21 winners to her six.
Keys also punished her friend's shaky serve, breaking her five times en route to victory.
"It's always tough having to play a friend but we've been doing it our whole lives at this point," Keys, 28, said.
"When we get on the court it's all business and when we get off the court we go back to being friends."
The 17th-seeded Keys will now face Wimbledon champion Marketa Vondrousova of the Czech Republic in the quarter-finals on Wednesday.
Ninth seed Vondrousova booked her place in the last eight with a battling win over unseeded American Peyton Stearns, coming from a set down to win 6-7 (3/7), 6-3, 6-2.
Vondrousova made history in July after becoming the first unseeded woman to win Wimbledon, her first Grand Slam title.
The 24-year-old has never been past the fourth round at the US Open, and admitted after her win on Monday she had surprised herself by advancing to the last eight.
"She was playing great from the beginning, and I just tried to stay in the game," Vondrousova said of Stearns.
"I'm very happy. I actually didn't expect it after Wimby -- it was a lot of pressure. We'll see what happens next."
The identities of two other women's quarter-finalists will be decided later Monday, with second seed Aryna Sabalenka -- destined to be the new women's world number one after Swiatek's exit on Sunday -- facing Russia's Daria Kasatkina.
Tunisia's fifth-seeded Ons Jabeur faces China's Zheng Qinwen in the other women's fourth round clash.
In the men's draw meanwhile, current world number one Alcaraz aims to advance to the quarter-finals when he faces Italy's surprise package Matteo Arnaldi.
Arnaldi, the world number 61, reached the last 16 after upsetting British 16th seed Cameron Norrie in straight sets.
That has now given Arnaldi a chance of a dream shot against Alcaraz.
"When I saw the draw, that was my goal, to go and play against him," said Arnaldi of the prospect of playing Alcaraz.
"I don't want to say we play very similar. But he's very physical, and I think I'm a bit physical."
In other men's matches on Monday, Britain's Jack Draper takes on eighth-seeded Russian Andrey Rublev while third seed Daniil Medvedev, the 2021 champion, faces Australian 13th seed Alex de Minaur.
In the late game on Arthur Ashe, Germany's Alexander Zverev, seeded 12, will aim to derail Italian sixth seed Jannik Sinner.
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