Aleksandra Krunic, who upset No. 3 seed Petra Kvitova to reach the fourth round of the U.S. Open, says she did not lack facilities and resources growing up, which might have delayed her progress as a player.

Though it has usually been a challenge for Serbian players to access top-level training in the country, Krunic was born in Moscow and trained in Russia until she was 17. She then moved to Slovakia and trained there until 2013, when she went to Serbia to live with her grandparents.

"I have a sponsor. It's my father's childhood friend," she said. "He's helping me since I'm 14, I think. I'm for him like a second daughter, second kid.

"I had everything, you know, on the plate for me."

That both helped and hindered the 21-year-old's progress, she admitted.

"You know, when you have everything, yeah, it's a big plus, but it can be also a big minus. When you have everything you don't know actually what you actually need.

"I was not used to fight for some things myself, because it was like, 'Yeah, here is Aleks, whatever it is, take it. Now go and play.' That was pretty tough subject for me, but I think I managed to, you know, get it all together and not think about these things and just listen to myself more."

Despite growing up in Moscow, she described herself as friendly with other Serbian players such as Jelena Jankovic, Bojana Jovanovski, and Novak Djokovic.

"Jelena, because we have shared a lot of good moments in the Fed Cup doubles. Yeah, you know, actually I kicked out Djoko from the quiet room in the gym before my match with Madison, and today before my match I also saw him in the gym. So, you know, we were kind of joking after the match. He said, 'Keep kicking me out if you're playing good, you know,'" she said.

Kvitova, is another friend of hers, though that did not prevent Krunic from defeating the two-time Wimbledon champion.