Top seed Caroline Wozniacki stayed on course for her maiden Grand Slam title on Monday with a 6-3, 6-4 win over Maria Sharapova in a pulsating fourth-round tie.
In the end, it was Sharapova's vulnerability on serve at key points that proved to be the main difference, handing vital breaks to the Dane through double faults in each set.
Wozniacki will play unseeded Dominika Cibulkova of Slovakia for a place in the semi- finals.
"It was a tough match. She is a great player and I knew it wasn't going to be easy," last year's runner-up said.
"I got a lot of experience from last year and that helped me a lot today."
The world number two went into the match having dropped just three games in her first three outings in New York, the least in a Grand Slam event by any woman since Mary Pierce at the 1994 French Open.
She had also won three out of the last four WTA tournaments in the build-up to New York and had enjoyed a 17-1 win-loss record since Wimbledon.
But Sharapova, three years older at 23, had won their two previous meetings, both coming in 2008, and, after battling back from injury, had been showing glimpses of her best form in the last few weeks.
The match was expected to come down to Sharapova's explosiveness against Wozniacki's consistency, and so it turned out in the opening few games with the Russian facing the first break points in the second game before blasting her way out of trouble.
Two games later, though, Sharapova came out on top in a long, punishing rally to lead 40- 30 on her serve, but then flung in three straight double faults to hand the break to Wozniacki.
The Dane held serve for 4-1 and had two further break points on Sharapova's next serve, but failed to convert them.
In the following game, Sharapova let slip five break points of her own, but finally converting at the sixth attempt using her big forehand to pound a hole in Wozniacki's solid defenses.
But once again, the Russian struggled on serve and was broken to love. This time Wozniacki held firm to take the set 6-3 on her third set point in 53 minutes.
In what was a top-quality encounter, both players battled hard to hold on to their serves in the second set and it was Wozniacki who found an extra gear in the seventh game to grab the break of serve she needed thanks to another double fault from Sharapova.
Wozniacki then served out twice to ensure one of the biggest wins of her career and clear her path for another run at the final in Flushing Meadows.