NEW YORK (AFP) — Defending champions Jannik Sinner and Aryna Sabalenka are chasing rare repeat US Open triumphs as the final Grand Slam of the season gets underway in New York on Sunday.

Women's world No. 1 Sabalenka opens the defense of her 2024 crown on the Billie Jean King National Tennis Center's Arthur Ashe Stadium showcourt against unseeded Swiss player Rebeka Masarova in one of the highlights of the opening day of the singles, which is being held on a Sunday for the first time in history.

Italy's world No. 1 Sinner, meanwhile, begins his bid for back-to-back titles on Monday, when he takes on unseeded Czech Vit Kopriva.

Both Sabalenka and Sinner are attempting to become the first players to successfully defend their titles for more than a decade.

No woman has repeated at the US Open since Serena Williams won the last of a hat-trick of titles in 2014 following wins in 2013 and 2012.

On the men's side, it has been 17 long years since a successful title defense, with Roger Federer's 2008 victory capping a remarkable run of five straight championships.

Sabalenka believes difficulty in repeating is part of the challenge of Flushing Meadows fortnight, where everything is noisier, flashier and simply bigger than other Grand Slams.

"I think it's a lot of pressure, definitely," the Belarusian said.

"Just because this place is so big, and it feels bigger than the other slams in some ways.”

"Maybe every time the defending champions come, they put so much pressure on themselves. But I feel like I'm experienced enough to just focus on myself and try to replicate that result."

Asked for her thoughts on the fact that the last 11 US Open titles have been split by 10 players, Sabalenka quipped: "My thought is to change it."

Sinner echoed Sabalenka's comments, attributing the difficulty of repeating in New York to a combination of factors — season-ending fatigue, scheduling of games and size.

"We are heading towards the end of the season, so some players, they are tired," Sinner said.

"It's also the last big trophy of the year. Things can change... I don't know what's gonna happen this time. But of course it's a very, very difficult tournament to play."

Sinner and Sabalenka are the top seeds in a field that will be rewarded with a record $90 million purse, making the US Open the richest tournament in the world. The men's and women's singles champions will receive $5 million each.

The biggest threat to Sinner's hopes of defending his title is likely to come from Spain's second seed Carlos Alcaraz.

The 22-year-old, winner of the US Open in 2022, has already faced Sinner twice in Grand Slam finals this season, winning a five-set classic in the French Open in June before losing to the Italian in the Wimbledon final last month.

"Every time we face each other on court, we raise the level to the top," Alcaraz said on Saturday as he prepared to face unseeded American Reilly Opelka in the first round on Monday.

The men's draw will also see former world No. 1 Novak Djokovic take another tilt at winning a record 25th Grand Slam.

Djokovic, 38, has not played since reaching the semi-finals at Wimbledon last month, where he was comprehensively beaten by Sinner.

Back in the women's draw, Sabalenka will be wary of threat posed by second seed Iga Swiatek, who warmed up for New York with victory at the Cincinnati Open, which in turn followed a breakthrough win at Wimbledon.