Camila Giorgi

Camila Giorgi of Italy celebrates her match win over Caroline Wozniacki of Denmark during the Connecticut Open at the Connecticut Tennis Center at Yale on August 20, 2014 in New Haven, Connecticut. (Elsa / Getty Images / August 20, 2014)

NEW HAVEN — On Monday, Caroline Wozniacki said she felt like she was in great shape. She's training to run in the Nov. 2 New York City Marathon and her fitness is the best it's ever been on the tennis court.

But even top marathoners get tired. And Wozniacki, who said she hasn't had a day off since she arrived in Montreal a little over two weeks ago, looked sluggish and spent on Stadium Court Wednesday afternoon against Camila Giorgi of Italy.

Giorgi, ranked 38th in the world, sent fourth-seeded Wozniacki to her earliest defeat in New Haven at the Connecticut Open, beating her 6-4, 6-2 in a second-round match. Giorgi will face Spain's Garbine Muguruza - who defeated Peng Shuai 6-3, 3-6, 6-3 Wednesday - in a quarterfinal in the third match on Stadium Court Thursday afternoon.

"The tank just wasn't full today," said Wozniacki, who won the tournament four times from 2008-2011. "When you play a player like that, you just don't get by by not moving 100 percent.

"She played well. My head was saying, 'Go for it,' but my legs were just going a little bit too late. I just didn't move as well as I usually do. I felt a little tired out there. It doesn't help when you play somebody where you have to be ready from the first point."

Giorgi, playing for the first time in the main draw at New Haven, took control from the start of the match, breaking Wozniacki in the first game and firing serves of up to 117 mph. She ended the match with a 109 mph ace.

"She played well and I didn't play very well," Wozniacki said. "I would have loved to have been here longer and try to defend the title again but it wasn't to be this year."

Wozniacki, a former world No. 1 who is now ranked 11th, lost in the semifinals of the tournament the past two years.

Giorgi hadn't lost a first round main draw match this year until she played her past three tournaments, when she lost in Bastad, Montreal and Cincinnati. So her 6-3, 6-1 first-round victory over Coco Vandeweghe Monday was big. But this was bigger.

"I think today the serve was the key," she said. "This is my game normally."

Asked if she could win the tournament if she keeps playing this way, she said, "Yes. I just need to play my game."

It wasn't the first time Giorgi had beaten Wozniacki, nor was it her biggest win against a highly ranked player. The two faced off at the U.S. Open last year, with Giorgi winning 4-6, 6-4, 6-3 in the round of 32, although Wozniacki did beat her in June at Eastbourne, 6-7 (7), 6-4, 6-2. Giorgi's biggest win was at Indian Wells in March when she came out of qualifying to beat then-No. 5 Maria Sharapova 6-3, 4-6, 7-5 in the first round.

Wozniacki said she didn't remember much about her loss to Giorgi at the Open but she did remember how aggressive the Italian player could be.

"She always plays like that," Wozniacki said. "I knew I had to be aggressive out there and try to push her back but I just didn't manage to do any of that today."

Wozniacki wasn't too down on herself after the loss. She said she would focus on the positives of the past few weeks, after advancing to the quarterfinals in Montreal and semifinals in Cincinnati, where she lost to Serena Williams both times.

"I just need to take the positives out of it," she said. "I've got a few more days before [the U.S. Open in] New York to rest up a little, practice."

And in an afternoon second-round match that lasted over three hours, until after 8 p.m., Belgium's Kirsten Flipkens outlasted Andrea Petkovic of Germany, 4-6, 7-6 (7), 7-6 (8).