NEW YORK (Reuters) - Rafael Nadal tried to put a brave face on it but the defending U.S. Open champion could not hide his frustration after yet another physical setback denied him the chance to add to his haul of Grand Slam titles.
It was evident early in a semi-clash with third seed Juan Martin del Potro that the world's number one player was hurting, but in true Nadal fashion he tried to persevere until retiring with knee soreness after dropping the first two sets.
"You could imagine very difficult for me to say goodbye before the match finish. But at some point you have to take a decision," Nadal told reporters. "It was so difficult for me to keep playing at the same time that way, having too much pain.
"That was not a tennis match at the end. It was just one player playing, the other one staying on the other side of the court."
Nadal, who reached the last four in New York on the back of brilliant shot-making and incredible movement, looked out of sorts early against Del Potro and struggled to cover the court with his usual tenacity.
While leading 4-3 in the first set Nadal got his right knee taped, before later ripping it off and having it re-taped early in the second during a medical timeout in which he also had his leg massaged.
Nadal tried to play through the pain but when Del Potro blasted a forehand winner to go ahead 7-6(3) 6-2 the Spaniard removed his headband and said he could not continue.
The 32-year-old, who also retired from the quarter-final in Australia this year with a muscle strain in his right thigh, called the latest setback more aggressive than ones in the past in that it came on suddenly rather than progressively.
"This year have been a fantastic year until this moment," said Nadal, who counts the French Open among his five titles this year. "I lost four matches. Two of them I had to retired. Negative thing is two of them have been in quarter-finals and semi-finals of a Grand Slam, close to fight for titles.
"That's frustrating. Can't tell you another thing. Is tough for me."
Many experts have long felt Nadal's physical style of play, which relies on power and athleticism in contrast to his more graceful rival Roger Federer, puts him more at risk of injury.
But Nadal, whose 17 Grand Slam championship trails only Federer's 20, shrugged off that notion.
"All my career everybody say that because of my style, I will have a short career. I still here. I still here because I love what I am doing," said Nadal.
"I still have the passion for the game. I going to keep fighting and working hard to keep enjoying this tour and keep having chances to compete at the highest level. So that's all."
(Editing by Peter Rutherford)