World number one Williams, winner of the past two titles at Flushing Meadows and five overall, will try to match Chris Evert and Martina Navratilova on 18 career major titles when she faces her good friend Wozniacki tomorrow.
The 10th-seeded Dane was leading 7-6 (7/1) 4-3 when China's Peng Shuai dramatically retired from their semi-final with heat related illness.
Peng was taken from the court in a wheelchair as Wozniacki, leading 7-6 (7/1), 4-3, was declared the victor.
After some discussion she was helped form the court and granted a confusing and controversial mid-game medical timeout as Wozniacki, facing a break point, was left to try to stay loose hitting practice serves.
Peng returned to the court and after a break of 10 minutes played five more points before she was stricken again, finally crumpling to the court, her Grand Slam dream in tatters.
"It was very difficult to watch," said Wozniacki, who went to Peng's side when she fell to the court, offering her a comforting pat.
"Tennis is great but the health is more important. I wanted to make sure that she's OK," Wozniacki said.
Wozniacki was especially concerned knowing that Peng had surgery to correct a heart defect as a youngster.
She wasn't too worried about whether the extended break broke any rule.