NEW YORK -- The number of American women into the quarterfinals of the US Open is down to three after No. 2 Sofia Kenin was upset Monday night, hours after Serena Williams advanced in her quest for her 24th major title.
No. 16 Elise Mertens knocked off Kenin to earn a berth in the quarterfinals at Flushing Meadows for the second year in a row.
Kenin won the Australian Open and was trying to become the first back-to-back Grand Slam winner since Naomi Osaka won the US Open and the Australian in 2018-19.
Kenin had 26 unforced errors to only seven for Mertens.
Mertens will face Victoria Azarenka, who orchestrated an upset of her own on Monday, beating No. 20 Karolina Muchova 5-7, 6-1, 6-4. This will be her first quarterfinal appearance at Flushing Meadows since 2015.
Earlier Monday, with her breathing loud enough to hear in an empty Arthur Ashe Stadium and her third-set deficit a point from growing to 3-1 against someone who beat her less than two weeks earlier, Williams scrambled to extend a 13-stroke exchange until her opponent netted a forehand.
"Keep fighting!" Williams exhorted herself.
Locked in a tough fourth-round match Monday, without the benefit of a pro-American audience, Williams provided her own encouragement along the way to come back and beat 15th-seeded Maria Sakkari of Greece 6-3, 6-7 (6), 6-3 and reach the quarterfinals in a 12th consecutive appearance at Flushing Meadows.
"I feel like I'm pretty vocal with or without a crowd. ... I'm super passionate. This is my job. This is what I wake up to do. This is what I train to do, 365 days of the year," said Williams, who moved a step closer to a record-tying 24th Grand Slam title.
"Yeah, I'm always going to bring that fire and that passion and that 'Serena' to the court."
Williams joins fellow Americans Jennifer Brady and Shelby Rogers in the quarterfinals.
When the match ended, after Williams collected six of the final seven games, she turned and yelled toward her husband, who stood at his front-row seat and yelled right back.
How tight was this contest? Sakkari produced more aces than Williams, 13-12, and more total winners, 35-30.
Williams was two points from victory at 6-all in the second-set tiebreaker, but she faltered there, sending a backhand return long to give Sakkari her fifth set point and then pushing a forehand out.
But as is so often the case, when the outcome was hanging in the balance in the third set, which Sakkari led 2-0 but couldn't quite get to 3-1, Williams was better down the stretch, when it mattered the most.
"I have to be deadly honest: I wasn't brave enough in the third set. I kind of like, not choked, but didn't [convert] my chances," Sakkari said. "And if you don't get your chances with a good Serena against you, it's done."
This was a rematch from Aug. 25, when Williams faded after building a lead and lost in three sets to Sakkari at the Western & Southern Open, a hard-court tournament usually held in Ohio but moved to the US Open site as part of a two-tournament "controlled environment" without spectators amid the coronavirus pandemic.
"Of course I thought about [the loss], but ever so little because it's a completely different match, completely different scenario, completely different moment," Williams said.
In the earlier match, Williams' legs were cramping by the end, and she blamed herself for that situation, memorably declaring: "I put myself in a bad situation. It's like dating a guy that you know sucks."
Asked about that line Monday, Williams said, "Thank God, I got rid of that guy. Never want to see him again. He was the worst."
That setback made the American 3-2 since tennis returned from its COVID-19 hiatus, all three-setters. Since then, she is 4-0 at the US Open.
Williams, who turns 39 in less than three weeks, will face unseeded Tsvetana Pironkova for a semifinal berth.
Playing in her first tournament in more than three years after taking time off to have a child, Pironkova, 32, advanced with a 6-4, 7-6 (5), 6-3 victory over Alize Cornet.
Williams first won the title in New York in 1999 as a teenager and has six trophies here. In her most recent 11 trips to Flushing Meadows, Williams has four championships, three runner-up finishes and three semifinal losses.
She lost in the final in both 2018 and 2019, part of a stretch in which she has been to the title match at four of the past seven Grand Slam tournaments, falling just short of getting that elusive 24th, which would tie her with Margaret Court for most in tennis history.
Unlike Court's, all of Williams' major championships have come in the professional era.
"I always compete," Williams said when asked what about her work in this tournament has pleased her the most, "but competing and having a good attitude, I think, is what I'm most satisfied about."
Information from The Associated Press was used in this report.