NEW YORK—Flavia Pennetta, who won the U.S. Open but will retire at the end of the year, admits that she never believed she could triumph at a major.

Until last week, the 33-year-old had reached the semifinals of a major on just one previous occasion (the 2013 U.S. Open). Pennetta had won 10 career singles titles before the U.S. Open, including Indian Wells, and went on to beat No. 2 Simona Halep in the semifinals and Roberta Vinci in the final at Flushing Meadows.  
“We have this conversation two months ago with my physio and my [coach],” Pennetta said in response to a question from TENNIS.com, speaking to a small group of press following her victory. “They ask me, do you think you can win a Grand Slam? I say no. So now... They are laughing so much because they say, 20 days ago, you say no? And now we have the trophy.”  
Pennetta added that even though she played extremely well and could challenge for titles if she continued playing next season, she can no longer push herself mentally to compete. This year, outside of the U.S. Open, she has only reached the quarterfinals at two tournaments, Indian Wells and Dubai.  
“I think if you play 24 weeks, 20 weeks—you need always the same condition for fighting, and you have to feel all the time you want that so badly,” Pennetta said. “In the moment when it's not like that any more, when it's coming and it's going, there is some thing[s] you want to do, like Fed Cup, you are playing for your country so you're really motivated to do that, you play U.S. Open, so you are really motivated. But there are some tournament where I don't feel this special any more. And that's the point—when you don't feel always, something has changed.”

Pennetta plans to play Wuhan, Beijing and, if she qualifies, the WTA Finals.