Nadal vows passionate return to U.S. Open

"The passion is there," Nadal told reporters on Wednesday before taking the court against Australian Lleyton Hewitt at a fundraiser to support young New York area tennis players at John McEnroe's tennis academy on Randall's Island.

The 14-times grand slam winner has claimed three lesser titles this year but has disappointed in the biggest events and seen his world ranking slip to eight, the lowest he has been since he won his maiden major at the French Open a decade ago.

After losing to compatriot Feliciano Lopez in his second match in Cincinnati last week, Nadal sounded almost accustomed to a string of results that includes two losses to men outside the top 100 and a 13-10 mark against those in the top 30.

"Losing matches, that's part of my career, too," Nadal said, adding that losing was something "I have to accept now."

Nadal, however, has been upbeat in the run up to the U.S. Open, which starts on Monday at Flushing Meadows and is his last chance to continue his run of winning at least one grand slam title in 10 straight years.

"My feeling today is that I am playing much better than most of the time in the season," said Nadal, who has been relaxed and playful around town this week, playing a form of strip tennis against some Tommy Hilfiger models in a midtown Manhattan park to promote a new line of underwear.

"I have been losing matches that I should win. During the season there are some key moments that change the dynamic and I lost all the opportunities to change the dynamic in those key moments."

The two-times U.S. Open winner (2010, 2013), whose unrelenting muscular effort on court was a hallmark of his success, sounded confident about a turnaround in his return to Flushing Meadows after missing last year with a wrist injury.

"I am happy with the way I am practicing and I'm feeling well I can say," said Nadal, stressing he has been injury free.

"Now I need to win. And I need to be impassioned.

"I need to keep working with the same intensity and passion and I believe the results will come back."