Elena Rybakina has been bundled out of the first round of the Silicon Valley Classic by Daria Kasatkina in her first game since her Wimbledon triumph last month.
Rybakina won the first set comfortably before the Russian hit back for a remarkable turnaround win, 1-6 6-2 6-0 in one hour and 40 minutes.
Seventh seed Kasatkina was the runner-up at last year's Silicon Valley Classic and found her groove to break her Rybakina six times in the second and third sets.
Rybakina sent down three of her four aces for the match in the first set, but Kasatkina adjusted to her serve and won 52.1 per cent return points for the match.
Eighth seed Karolina Pliskova fought back from a slow start to get past Katie Boulter 1-6 7-6 (7-5) 6-3, despite 16 double faults.
American Amanda Anisimova overcame compatriot Ashlyn Krueger 6-2 7-6 (7-5). US pair Madison Keys and Taylor Townsend were also winners.
Third seed Simona Halep and top seed Jessica Pegula were both among the winners in the first round at the Citi Open in Washington DC.
Two-time major winner Halep cruised past Spanish qualifier Cristina Bucsa 6-3 7-5 in one hour and 20 minutes.
Halep, playing her first match since her Wimbledon semi-final loss to Rybakina, fired 19 winners but also 33 unforced errors.
Defending champion Pegula made light work of countrywoman Hailey Baptiste 6-2 6-2 in 73 minutes.
Pegula will face unseeded Australian Daria Saville, who won in three sets over Mirjam Bjorklund, while sixth seed Kaia Kanepi will meet China's Zhu Lin after both won on Monday.
Fifth seed Elise Mertens was a first-round casualty, going down 7-6 (7-4) 6-4 to Ludmilla Samsonova, while Croatian Donna Vekic knocked off seventh seed Mayar Sherif 6-4 6-1.
Six-time major champion Venus Williams played her first singles match in nearly a year but bowed out 4-6 6-1 6-4 to Rebecca Marino.
Sinner avoids O'Connell upset at US Open
Jannik Sinner beat Australia's Christopher O'Connell in straight sets to reach the fourth round of the US Open on Saturday, avoiding a similar fate to Carlos Alcaraz and Novak Djokovic.
The men's draw at Flushing Meadows has been blown wide open by early exits for Alcaraz and Djokovic, but the top seed remains in the hunt after a composed performance.
He reached the fourth round for a fourth consecutive year with a routine 6-1 6-4 6-2 victory, setting the tone with several huge groundstrokes in the opening set.
O'Connell never looked likely to fight back as Sinner smashed 46 winners in a ruthless performance, wrapping up his win in one hour and 56 minutes.
The Australian Open champion will face either Tommy Paul or Gabriel Diallo for a place in the last eight, and he was relieved to survive an opening week full of surprises.
"Everything can happen in this sport, so I try to stay on my side of the net and I'm trying to take the tournament day by day," Sinner said in his on-court interview after the win.
"Each opponent is a very tough challenge and [I am] also enjoying every moment on the court. There have been already a couple of upsets, so let's see what's coming.
"But I'm very happy to still be here and hopefully I can play as many matches as possible. But as I said, we go day by day and then we'll see how it goes."
Data Debrief: Sinner heating up
Sinner lost his first set of the tournament to Mackenzie McDonald but has been largely untouchable since then.
He has only lost 18 games across his last nine sets of tennis, an average of two per set.
The Italian's winning streak now stands at seven straight matches, meanwhile, following his triumphant Cincinnati Open run.
US Open: Sabalenka overcomes first set scare to progress in late start
Aryna Sabalenka came from a set down to keep her US Open dream alive as she beat Ekaterina Alexandrova in the early hours at Flushing Meadows.
Sabalenka emerged on court at the Arthur Ashe Stadium at seven minutes past midnight following Novak Djokovic's shock exit from the tournament, and consecutive upsets looked to be on the cards.
However, the Belarusian responded from a disappointing opening set as she looks to avenge her defeat in the showpiece match against Coco Gauff a year ago.
The world number two set up a meeting with Belgium's Elise Mertens as she emerged a 2-6 6-1 6-2 victor in one hour and 40 minutes in New York.
"I don't know what I did, I was just trying to stay low, put as many balls as I can back on that side," Sabalenka said.
"She played incredible tennis in the first set. It was really tough to change it."
"I'm really happy I was able to turn around this match and get this really difficult win. Thanks so much for staying that late."
Data Debrief: Sabalenka survives late, late show
Sabalenka's encounter with Alexandrova was the latest starting match at the US Open, beating Gabriela Sabatini and Beverley Bowes' meeting in 1987 that started at midnight.
The Belarusian, however, will be hoping for a much improved performance in the next round, having produced 27 unforced errors throughout the contest and serving four double faults.
US Open: Djokovic played 'worst tennis ever' in Popyrin loss
Defending US Open champion Novak Djokovic suffered a shock exit at Flushing Meadows as he lost in four sets to Alexei Popyrin.
Djokovic arrived at the tournament with confidence following his gold medal triumph at the Paris Olympics, and was tipped to lift a record-extending 25th grand slam title.
However, the Serbian's hopes of a fifth success in New York were dashed as he followed fellow favourite Carlos Alcaraz out of the competition in the third round.
Popyrin emerged victorious in three hours and 18 minutes, prevailing 6-4 6-4 2-6 6-4 to confirm Djokovic's earliest exit from the tournament in 18 years.
"I have played some of the worst tennis I have ever played," Djokovic said.
"Honestly, the way I felt and the way I played from the beginning of this tournament, third round is a success."
"I tried my best, I didn't have any physical issues. I just felt out of gas, and you could see that. From the first match, I just didn't find myself on this court.
"Life moves on. I'll try to recalibrate and look forward to what's next."
Data Debrief: Another one bites the dust
Djokovic was far from his clinical best, serving a career-high 14 double faults in a grand slam match and committing 49 unforced errors.
The Serbian will also end the year without winning a grand slam title, the first time he has done so since 2017.
But the day belonged to Popyrin, saving 12 of the 16 break points he faced against the world number two, finding particular success with his forehand as 31 of his 49 winners came from that shot.
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