Czech 10th seed Karolina Pliskova outplayed Serena Williams to reach the US Open final and end the American's reign as world number one.
Williams, 34, double-faulted on match point to lose 6-2 7-6 (7-5), and Pliskova will face German second seed Angelique Kerber in the final.
The result guarantees that Kerber will overtake Williams at the top of the rankings after the US Open.
Williams had also hoped to win an Open-era record 23rd major singles title.
She remains tied with Steffi Graf for both Grand Slam titles, and the WTA record of 186 successive weeks as world number one.
"Karolina played great today," said Williams, who revealed she had been hampered by a sore left knee since earlier in the tournament
"I think if she had played any less then maybe I would have had a chance. So I think I wasn't at 100%, but I also think she played well. She deserved to win today."
Australian Open champion Kerber beat Denmark's Carolina Wozniacki 6-4 6-3 to reach her first US Open final.
"It's just incredible, it's a great day," said Kerber. "To be here in the final for the first time means a lot and to be number one in the world - it sounds amazing."
bbc.com