Agnieszka Radwanska said she was too slow against teenage sensation Ana Konjuh after the Polish fourth seed lost in the fourth round of the US Open.
The overwhelming favourite with a quarter-final berth on the line in New York on Monday, Radwanska was stunned 6-4 6-4 by the 18-year-old Croatian.
Radwanska led 2-0 in the opening set but the 2012 Wimbledon finalist struggled from that point as Konjuh - a junior champion at Flushing Meadows - worked her experienced opponent around the court.
Speaking afterwards, Radwanska said: "She was playing pretty good tennis, serving very well. I just couldn't do any more I guess today.
"I was really trying. I was just too slow today. I was just trying to mix it up and not play in one spot.
"Sometimes it is difficult when the ball is going really big from the other side. So it's hard to control, really put the ball where you want. That's what I was trying to do. But, well, she was really solid."
Radwanska, who has never advanced beyond the fourth round of the US Open, continued: "Definitely it was only one break in the beginning, then I was not really close to break again.
"Maybe there was one or two chances. Then I didn't really hold my serve. I didn't serve good enough. Then, well, I'm here not as a winner."