NEW YORK (AP) Trying to serve out the biggest victory of her career, 18-year-old Ana Konjuh was startled - right along with everyone else under the roof in Arthur Ashe Stadium - by a piercing noise coming from the U.S. Open arena's sound system during a ball toss.
The Croatian recoiled, let the ball drop and put her left hand to her chest. Once Konjuh realized nothing bad was going on, she smiled, then resumed playing, just as calm and collected as ever, and finished a 6-4, 6-4 upset of No. 4 seed Agnieszka Radwanska on Monday night to reach her first Grand Slam quarterfinal.
''I just said to myself, `Stay in the game. Don't rush,''' Konjuh said.
Yes, she acknowledged later, she was scared for a moment by the odd sound. But nothing really threw her off her game on this night. Not the stakes or the setting - playing with the roof closed atop the largest tennis stadium she'd ever set foot in.
And definitely not Radwanska, the 2012 Wimbledon runner-up.
''I'm real excited about my performance tonight. My serve was pretty good tonight,'' Konjuh said. ''I don't think she had any answers.''
Konjuh, who is ranked only 92nd, was the junior champion at Flushing Meadows in 2013.
A fearless big hitter, Konjuh compiled a 38-9 edge in winners against Radwanska, who is 27.
Radwanska is now 0-5 in fourth-round matches at the U.S. Open, the only Grand Slam event where she hasn't been to the quarterfinals at least once.
''I just couldn't do any more, I guess, today,'' Radwanska said. ''I was really trying. I was just too slow today.''
She wondered aloud at her news conference why the match was played with the new $150 million retractable roof atop Ashe shut. A shower briefly had interrupted the previous match, Andy Murray's victory over Grigor Dimitrov, just before its final game, but then the wet weather quickly disappeared.
Before Konjuh vs. Radwanska began, the cover was closed atop the arena.
Asked whether she was told why by a tournament official, Radwanska said: ''Well, the risk of rain. But, well, there was always risk of rain, I guess.''
Would she have preferred for the match to be played without the roof?
''I don't know what would happen if that would be open. Sometimes,'' she added, ''I just don't understand why they're closing the roof.''
Konjuh didn't care, even saying she was ''real happy'' she got a chance to play with it shut.
She said the conditions suited her better than Radwanska.
''I didn't have any wind or sun or anything that's going to distract me,'' Konjuh said, ''so I just played my game and gave my best.''
This year at the All England Club, Konjuh held three match points against Radwanska but lost their second-round match 9-7 in the third set after accidentally stepping on a tennis ball and injuring her right ankle during a point at 7-all.
Konjuh next faces 10th-seeded Karolina Pliskova, who eliminated Venus Williams in three sets. Pliskova, who is 24, has never played in a major quarterfinal, either. Wednesday's other quarterfinal will be Williams' sister, No. 1 seed Serena, against No. 5 Simona Halep.
The quarterfinals on the other half of the draw are Tuesday: No. 2 Angelique Kerber vs. No. 7 Roberta Vinci, and Caroline Wozniacki vs. Anastasija Sevastova.
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