Stan Wawrinka's ATP Tour return at the Monte-Carlo Masters ended in defeat, while Jannik Sinner came through a tense encounter with Borna Coric.
The former world number three made his comeback after more than a year out with a left foot injury that required two surgeries but fell to a 3-6 7-5 6-2 defeat to Alexander Bublik in the first round.
Wawrinka, who entered the tournament as a wildcard having slipped to 236 in the world rankings, came from a break down to take the opening set but fatigue eventually set in and he was eliminated after two hours and seven minutes.
"I think the body's getting much better," said Wawrinka, who lost in straight sets to Elias Ymer at a Challenger tournament in Marbella two weeks ago.
"I'm still far away from where I want to be, but I think I'm [going in] the right direction. I think today was a positive match. It was a tough loss, of course, but I'm happy with the way I was playing today.
"I was playing better, feeling better on the court physically. I spent a lot of mental energy to focus, to stay there as it was tough at the end. But I'm happy with what I have done."
Ninth seed Sinner was taken the distance by Coric before completing a 6-3 2-6 6-3 victory to advance to the second round.
Sinner took a medical timeout in the second set while he dealt with some apparent abdominal pain, but he was able to battle back and take the win.
Diego Schwartzman needed over three hours to come from behind and defeat Karen Khachanov 6-7 (5-7) 6-3 6-3, while his fellow Argentine Federico Delbonis set up a clash with second seed Alexander Zverev by besting Jaume Munar 6-4 3-6 6-4.
Reigning champion Stefanos Tsitsipas faces a meeting with Fabio Fognini in the second round after the Italian overcame Arthur Rinderknech 7-5 4-6 6-3.
There were also wins for Hubert Hurkacz, Pablo Carreno Busta, Marin Cilic, Alex de Minaur, Albert Ramos-Vinolas and Sebastian Korda.
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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