NEW YORK — The environment at the U.S. Open is unlike anything tennis players deal with the rest of the year. There’s constant noise. The weather can go from brutally hot and humid to windy and cold. There’s even, on certain courts, a whiff of marijuana from a nearby park − which is legal these days in the city.

Some players handle these elements well. Others struggle year after year to feel comfortable playing at the Billie Jean King National Tennis Center. 

But the uniqueness of this venue − which feels more like a big, loud, American sporting event than your standard tennis tournament − is what makes it special. 

And the second week of the U.S. Open seems poised to deliver some classic matchups and highlight some of the game’s biggest stars. 

No matchup would be more anticipated than a final between 20-year-old defending champion Carlos Alcaraz and Novak Djokovic, who is trying to win his fourth U.S. Open and record 24th Grand Slam title. Those two played a five-set classic at Wimbledon that went in Alcaraz’s favor, then met again in Cincinnati two weeks ago where Djokovic won a third-set tiebreak after nearly four hours of extremely physical tennis. 

And on the women's side, the attention will now turn to whether either of the American stars, Coco Gauff or Jessica Pegula, can break through for their first major or whether No. 1 Iga Swiatek can repeat as champion and send the message that she’s still the world’s dominant player. 

Round of 16:Why Coco Gauff vs. Caroline Wozniacki is the must-see match of the US Open

With the round of 16 set to begin Sunday, here are five storylines to watch for the second week: 

Still on track for dream men's final 

There was a brief moment Friday night when it looked like Djokovic was in trouble against fellow Serb Laslo Djere. But for Djokovic, falling behind two sets to none against a vastly inferior player usually just gives him a clearer sense of how far the finish line is. Once Djokovic broke serve to start the third set, he had complete control of the match and seemed in no danger of getting upset, losing just five games the rest of the way. 

Alcaraz has rolled into the second week, dropping just one set to Dan Evans in the third round and looks to be playing himself into good form. 

It certainly seems likely Djokovic and Alcaraz will play a second consecutive Grand Slam final, but on paper Djokovic has fewer obstacles to get there. Though Alcaraz shouldn’t have much trouble in the next round with Italian long shot Matteo Arnaldi, he’s on a quarterfinal collision course with Jannik Sinner, whose heavy hitting game has given him problems in the past. Last year, those two played for 5 hours, 15 minutes in the quarterfinals before Alcaraz emerged victorious. There’s also a potentially tough semifinal looming against No. 3 seed and 2021 champion Daniil Medvedev. 

Coco Gauff's big tests are coming now  

In two of her three matches so far, the 19-year-old Gauff has started off a bit tight and gotten herself in trouble before making the right adjustments and running away in the end. That’s a good indicator her new coaching team, led by ESPN commentator and former top-10 player Brad Gilbert, is giving her the confidence to problem solve instead of panic after falling behind. 

But from here on out, Gauff’s margin for error gets much smaller. Her matchup Sunday with former No. 1 Caroline Wozniacki, who came out of retirement this summer, is going to require Gauff to hit a different gear and play a more aggressive match instead of getting into the longer, physical points that wore down her previous opponents. 

Then if she gets by that one, Gauff would be on track to face Swiatek, who has absolutely crushed her first three opponents. Gauff beat Swiatek in Cincinnati for the first time in eight tries, and there’s no doubt Swiatek will come out looking for revenge in the quarterfinals − if she gets there. 

Swiatek will have her own issues to deal with Sunday against Jelena Ostapenko, who has won all three of their meetings. Ostapenko is an unpredictable player who goes for big shots all the time, and she’s a handful for anyone when she’s on. 

The U.S. will likely have a men's semifinalist again

Last year, Frances Tiafoe lit the U.S. Open on fire when he made a semifinal run, becoming the first American man to get that far since Andy Roddick in 2006.

Despite playing pretty poorly this summer by his standards, there’s a great chance Tiafoe can repeat that feat. But if he doesn’t, it’s likely because he got beat by another American in the quarterfinal. 

Tiafoe will be heavily favored to advance against Australian wildcard Rinky Hijikata on Sunday afternoon. And if he does, his quarterfinal opponent will be either No. 14 seed Tommy Paul or Ben Shelton, the former NCAA champion out of Florida who has made an immediate splash in his first year on tour. 

Those two played in the Australian Open quarterfinal with Paul winning in four sets. He’s had an excellent hard court season, including a win over Alcaraz in Toronto.

After struggling to string wins together following his breakthrough in Australia, Shelton’s game has come alive in New York. He’s been bombing first serves, including one in the third round that clocked in at 147 mph. Shelton is an explosive athlete with a showman’s streak who leans into the rowdiness of the New York crowd. If he were to meet Tiafoe in the quarterfinals, it might be the most electric atmosphere of the tournament. 

Sickness is an X-factor

In her pre-tournament press conference, last year’s U.S. Open finalist Ons Jabeur was noticeably having sinus issues. But when asked, she laughed it off as a situation where  “American AC kills me.” 

When she got on the court, however, Jabeur’s physical issues were clearly more significant than an adjustment to air conditioning. Somehow, she managed to hack and cough her way through the first couple rounds despite being noticeably low on energy.

“I'm taking a lot of medicine,” she said Thursday. “I'm doing, like, everything I can with my team trying to recover. Honestly, they have amazing doctors here, so they've been helping me. Every day I'm there. Basically took every medication they have.”

It seems there’s some kind of mystery bug going around the U.S. Open grounds (the tournament does not require COVID testing). Several players have been sick, including former champion Dominic Thiem, who felt so bad he had to retire after a set against Shelton. Hubert Hurkacz was sick and ran out of gas in the second round. Chris Eubanks seemed to be struggling with some type of stomach issue in his loss to Benjamin Bonzi and asked for medical attention at one point. And in his second-round match, Nicolas Jarry could be seen blowing his nose during changeovers and was wearing one of those breathing strips to help open his nasal passages. 

Nobody likes to talk about sickness or make excuses, but it’s been noticeable. And if there’s really something going around the locker room, there’s certainly a chance that one of the contenders could fall ill and impact who wins the tournament. 

Another chance for Jessica Pegula 

Asked about what it's going to take for her to reach the semifinals of a Grand Slam after going 0-for-6 in the quarterfinal round, Pegula was matter-of-fact about it before this tournament. 

“Just need to win a quarterfinal,” she said with a slight smile. “That would help me get past the quarterfinal stage.”

The No. 3 seed and top-ranked American, Pegula is going to have another good chance to break through. Fresh off a title in Canada a few weeks ago, she’s playing well enough. She has the right attitude, too. And the draw is certainly interesting, as she will face a good friend in No. 17 seed Madison Keys in the next round. There’s also a chance Pegula will get an opportunity for revenge in the quarters against Marketa Vondrousova, whom she led 4-1 in the third set of the Wimbledon quarterfinal before the match turned. Vondrousova went on to win the title, showing just how close Pegula was to getting a Grand Slam trophy.

As well as she is striking the ball and with the New York crowd behind her − Pegula is a native of Buffalo and her parents own the Bills and Sabres − this could be her moment.