Serena Williams' latest bid for a 24th Grand Slam singles title ended as Victoria Azarenka rolled back the years to reach the US Open final.
Williams, seeking to equal the all-time record for majors, raced away with the first set but lost 1-6 6-3 6-3 to the inspired former world number one.
The American called on the trainer for an ankle problem early in the third set and struggled physically after that.
Two-time Australian Open champion Azarenka had been appearing in her first Grand Slam semi-final since 2013 following troubled times on and off the court.