Iga Swiatek has carried the torch as world No.1 on the women's tour for more than a year but the drained defending US Open champion faces a major threat to her position with Aryna Sabalenka poised to pounce.
Swiatek has had a vice-like grip on top spot since inheriting it after Ash Barty's retirement in March 2022, but Australian Open champion Sabalenka has steadily chipped away at the deficit and can leapfrog her rival by winning in New York.
Swiatek has not been beaten before the quarter-finals of a tournament since January but the 22-year-old Pole has looked to be running out of steam ahead of the year's final grand slam.
"I would say my tank of fuel is pretty empty," she said after going out in the semi-finals at recent events in Montreal and Cincinnati.
"I'm happy I'm going to have days off."
The French Open champion said she would try not to be hard on herself ahead of her US Open title defence.
"Being a defending champion isn't easy," Swiatek said.
"I'm going to take it easy on myself and just try to do everything step by step."
Despite needing to recharge and refocus, Swiatek will still be heavily backed to retain her crown and secure her fifth grand slam title but Belarus's Sabalenka underlined her own hard-court credentials with her breakthrough win in Melbourne.
She followed that up by making the semi-finals at the French Open and Wimbledon, where she could have taken the No.1 ranking for the first time.
Sabalenka also enjoyed success in the US earlier in the year with a run to the Indian Wells final where she crashed to fellow big-hitter Elena Rybakina, another top Flushing Meadows contender despite recent health and fitness worries.
The trio have won five of the past six grand slams, sparking talk of a 'big three' era in women's tennis but several other threats have emerged for the US Open crown in the last few weeks.
Marketa Vondrousova heads that list after the former Roland Garros runner-up broke her grand slam duck at Wimbledon, while Jessica Pegula and fellow American Coco Gauff are primed for deep runs after triumphs in Montreal and Cincinnati respectively.
"I believe in just giving it your all," said former French Open runner-up Gauff, who beat Swiatek en route to winning her first WTA 1000 title.
"That's all you can do. I'm going to give it my all in the US Open. If things go great, that's exciting. If not, I go back and work hard and get ready for the next one."
World No.3 Pegula has raised US hopes of a first home-grown Flushing Meadows champion since Sloane Stephens in 2017 after she beat Swiatek in Montreal.
Ons Jabeur will hope to overcome a foot problem she suffered last week as she continues her quest for a first grand slam singles crown.
The Tunisian trailblazer lost in the 2022 US Open final and has been runner-up at Wimbledon for the last two years.
Former world No.1 Caroline Wozniacki, twice a beaten finalist at Flushing Meadows, is one of those to be awarded wildcards for the tournament.
The Dane returned to competitive tennis earlier this month after more than three years away to start a family.