A group of young men was sitting courtside at Arthur Ashe Stadium, close to Caroline Wozniacki’s chair. As her match with Madison Keys came down to the final games, each time Wozniacki won a point they would sing “Sweet Caroline.”

It had to be music to Wozniacki’s ears.

Continuing on what is her first roll of the season, Wozniacki beat Keys, 6-3, 6-4, Sunday to reach the quarterfinals of the U.S. Open. That’s the first time she’s been this far in a tournament since she made the quarterfinals in Auckland, New Zealand, at the start of the year.

Right ankle, left elbow and back injuries have greatly impacted her season. She lost in the first round of the Australian Open and at Wimbledon. She skipped the French Open with the ankle problem, and this summer had a poor hard-court season. Her ranking had fallen to No. 74. There wasn’t much reason to believe that she would go deep in the Open, except this is where she has had her greatest Grand Slam success. She enjoys the National Tennis Center and the city, where she has an apartment.

“I’m not surprised that I could make it this far because I always believe in myself and I always think that, you know, when I got on court I always believe that I’ll win the match,” said Wozniacki, ever the optimist.

The rising American star Keys at 21 is the No. 9 player in the world and the Open’s No. 8 seed. Her big serve and walloping forehand are formidable weapons. But Wozniacki is a cunning counterpuncher and she took what Keys delivered and returned to sender.

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“She plays the ball really strong. She has a great forehand, good serve,” Wozniacki said. “She was really hitting some bombs which I was really pleased the way I was able to return a lot of those serves back and just kind of getting those actual balls in play.”

Wozniacki returned serve impressively and countered Keys’ forehand, playing to her backhand frequently, where Keys was error prone. Wozniacki made only seven unforced errors, while Keys made 33. Wozniacki was credited with forcing another 24 errors, while Keys forced 16. Wozniacki also won 80 percent of her first serves while Keys won 67 percent, and only 38 percent of her second serves. Wozniacki constantly was forcing Keys’ service games to deuce.

“I think just overall I wasn’t very solid today,” Keys said. “Even the service games I won, a lot I was up 40-love, 40-15, and it was back to deuce and it was a tight game. I feel like I was making mistakes and then getting behind in the score and trying to hit winners just to kind of quickly get back into games.”

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Keys found that many of her good shots were coming back, a Wozniacki trademark when she’s on top of her game.

“She’s an amazing mover. She gets a million balls back,” Keys said. “I think she does an amazing job keeping the balls deep in the court, so you feel like you can’t dictate the point right away, which I think makes people go for things too soon.”

It can’t be too soon for Wozniacki to get back into the swing of top-flight tennis with a body that finally is cooperating.

“My body is ready to go through like a beating,” said Wozniacki, who is all too ready to give one.