The surprises just keep on coming.

The women's draw here has been blown apart by the likes of Simona Halep, Agnieszka Radwanska, Ana Ivanovic and Angelique Kerber already having been dispatched out of the tournament.

And yesterday Petra Kvitova joined that sorry list, the Wimbledon champion beaten comprehensively by Aleksandra Krunic, the world No 145 from Serbia who had come through qualifying to reach the fourth round in only her second appearance at a Grand Slam.

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It was a magnificent display from the 21-year-old, who triumphed 6-4, 6-4 to ensure the Czech No 3 seed's difficulties here at Flushing Meadows continue. She has never been past the fourth round in New York. In the previous round, Krunic had upset the American hope Madison Keys, but what is most startling about her run here is how the Serbian had failed to take a set off anyone in the top 30 prior to arriving in the Big Apple.

"I tried not to think about the score, and I kept telling myself leave it on her because everything is up to her still and she's in charge," she said. "I tried to put the pressure off my shoulders, because usually in the last couple of years I'm the one who is putting the pressure on myself. I didn't expect myself to be so calm, but I really focused 100% not to think about the court, about Petra.

"Coming through qualifying gives you a lot of confidence. Now I'm in the second week of a Grand Slam. I still cannot believe I won against Madison, and now I won against Petra. It's like so many things happening in my mind that I'm trying to stay on the ground, because I don't think it's ever going to happen to me.

"But I'm trying to believe it, but when I believe it I'm afraid that I'm going to have so many emotions. So I'm trying to keep it all together. My coach is already screaming and jumping, and I'm trying to stay away from him so I don't listen."

Krunic, who is the Serbian No 5, showed once again the strength of depth which makes the Balkan state one of the most powerful tennis nations in the game. Questions as to why Serbia have an embarrassment of riches, the likes of which Britain can only dream of, can wait for another day. But it is clear having the likes of Novak Djokovic and world No 10 Jelena Jankovic to call upon whenever she likes clearly helps Krunic.

"I'm pretty close with Jelena, because we have shared a lot of good moments in the Fed Cup doubles," she added. "And I actually kicked Djoko out from the quiet room in the gym before my match with Madison, and today before my match I also saw him in the gym. He said: 'Keep kicking me out if you're playing good!' They are being very supportive and helping me, Jelena helps me with tactics too which I appreciate a lot."

There were no problems for Victoria Azarenka, the No 16 seed, who crushed Elena Vesnina 6-1, 6-1, nor top-seed Serena Williams, who overcame her third compatriot, Varvara Lepchenko, 6-3, 6-3.

Steve Brenner