Wozniacki stunned by Giorgi
There will be a first time grand slam semi-finalist from the third quarter of the US Open draw after Italian qualifier Camila Giorgi stunned sixth seed Caroline Wozniacki in the night session on Arthur Ashe.
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Giorgi, 21, reached the fourth round at Wimbledon last year and she got her reward for taking the game to Wozniacki, hitting 46 winners to the Dane's 13 in a 4-6 6-4 6-3 victory.
Giorgi next plays 10th seed and countrywoman Roberta Vinci, who was a quarter-finalist last year, with the winner of that match to meet either Halep or Pennetta.
Wozniacki rued letting her chance slip away in the second set and felt there was little she could have done in the decider.
She said: "I had many break-point opportunities in the second I didn't take and all of a sudden she came into the rhythm and just started hitting everything and it was starting to go in.
"She put me under pressure and I started to play a little short, and then all of a sudden she was all over the ball. She took very high-risk shots and things were going in for her.
"I was really trying to fight for every point and fight as much as I could, and it just wasn't enough today."
Wozniacki has shown encouraging form in patches this season but it is the first year since 2008 that she has not reached the quarter-finals in any grand slam.
She said: "It wasn't the best year for me at the majors but there is next year.
"Overall I feel the progress has been good. I have been hitting the ball well and playing well. I hope there will be more positives in the end of the season."
Illness scuppered Petra Kvitova's US Open bid as she was easily beaten by American Alison Riske in the third round in New York.
Former Wimbledon champion Kvitova revealed after the 6-3 6-0 defeat that she has been struggling with a virus and will have more blood tests before returning home.
The seventh seed said: "Unfortunately I was lying in bed yesterday and I had a fever, so I didn't come to site yesterday.
"My body didn't help me today to move. I tried to play, I tried to fight, but my body wouldn't let me fight.
"It's unlucky at a grand slam. I will take it for this year and next year will be better."
Kvitova suffered from a similar problem at Wimbledon this year, where she lost in the quarter-finals to Kirsten Flipkens.
Riske, who was ranked outside the top 200 in February, has had a rapid rise.
The 23-year-old reached the third round at Wimbledon and will climb again from her current ranking of 81.
She said: "It was really emotional for me. It's just really cool because I feel like I'm playing within myself. I'm not doing anything crazy. It just feels really comfortable. It's exciting."
Riske is one of a whole group of young Americans taking women's tennis by storm, and it looked like Christina McHale would join her when she served for the match in the second set against 13th seed Ana Ivanovic.
But the Serb fought back to win 4-6 7-5 6-4 and next meets second seed Victoria Azarenka, who was also involved in a major battle against France's Alize Cornet before coming through 6-7 (2/7) 6-3 6-2.
Azarenka has won her last three meetings against Ivanovic but was pushed hard in the most recent match in Carlsbad last month.
Ivanovic is out for revenge and said: "I was really close I felt (in Carlsbad). I have a game that can challenge the top players. I look forward to that opportunity.
"That's why I'm also very happy to pull through matches like this. Now I have a day to recover, recuperate, get ready for that match.
"It's going to be tough. But I really want to take on that challenge as a positive thing."
Azarenka and Cornet's match was very similar to their last meeting at the same stage of the French Open, and the second seed was relieved to be through.
Azarenka said: "I don't think I played my best tennis today for sure, but I have to give her credit. She played really well.
"The dynamic of the match was a little bit weird for me. I felt like I didn't take my opportunities in the first set, which was kind of a waste and it gave her, I think, a lot of confidence and she really came up with the great shots when she needed with big serves.
"I've actually never seen her serve like that before, so that's definitely a big improvement from her side.
"I'm glad that I could turn things around in the second and third sets. There was more me playing, more finding the rhythm. I felt much better in the third set than in the first set."
Romania's Simona Halep continued her remarkable run by thrashing 14th seed Maria Kirilenko 6-1 6-0.
Halep, who came close to losing to Britain's Heather Watson in round one, has won four of her last seven tournaments and meets Italy's Flavia Pennetta in the last 16 after she knocked out former champion Svetlana Kuznetsova 7-5 6-1.
Riske plays Daniela Hantuchova, who saved four match points against Julia Glushko of Israel on her way to a 3-6 7-5 7-6 (7/4) victory.