The Dane was forced to retire from her New Haven semi-final against Maria Kirilenko after experiencing trouble with the injury she first picked up in her last-eight match.
Wozniacki's concession meant that she could not retain the title that she has won for the past four years, but she admitted that the pain was too much for her to continue.
"I could feel it from the start. But, you know, it just started to get a bit worse," said Wozniacki, girlfriend of Irish golfer Rory McIlroy.
"I decided to stop because if you don't feel 100%, you can't compete at 100%.
"It's better to let it rest. I wasn't going to win this match anyways if I'm not on 100% fit level."
Wozniacki, who is seeded eighth for the Flushing Meadows Grand Slam, is scheduled to face Romanian Irina-Camelia Begu in her first-round clash.