Third-seeded Petra Kvitova was the latest upset victim after qualifier Aleksandra Krunic pulled off the biggest shocker thus far of the U.S. Open beating the Wimbledon champion, 6-4, 6-4, on Saturday.

Krunic had never taken even single set from a top-30 player before her third-round upset, so she was surprised too at her accomplishment.

"Of course I didn't expect to win,'' Krunic said via The New York Post. "Now I'm in the second week of a Grand Slam. I don't know, I still can't believe I won against Madison [Keys], and now I won against Petra. It's so many things happening in my mind that I'm trying to [stay calm]. ... I'm trying to believe it, but then when I believe it I'm afraid I'm going to have so much emotions. So I'm trying to keep it all together.''

Krunic, 21, was stellar for much of the match as she upset the heavily-favored Czech. Kvitova was seeking to grab the world number one ranking from American Serena Williams with a second straight Grand Slam win. But she fell to the proverbial upset axe, making her the fifth of the top eight seeds to exit early in the year's final Grand Slam.

Kvitova said: "I'm very disappointed. I wanted to win, and unfortunately I didn't. She (Krunic) played really unbelievable tennis and she put a lot of balls back - almost all of them."

Looking back, Kvitova could have prevented the upset if it were not for her 34 unforced errors.

"I don't know. I think it's the women's tennis. I'm not pretty sure what's happening. If I see this year, for example, all the Grand Slams have a big upsets. It's same here. I think just the tennis is so close, the levels are very close, everybody really can beat everyone from the top, so it's really difficult to stay on the top,'' she added.

With the win, Krunic goes on to meet the winner of the Victoria Azarenka-Elena Vesnina match-up in the fourth round.