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PARIS Oct 31 (Reuters) - Rafael Nadal's participation in the Masters Cup hung in the balance on Friday after the world number one retired with a knee injury in his Paris Masters quarter-final against Russian Nikolay Davydenko.
The tournament trainer massaged the Spaniard's right knee and thigh at the change of ends when 4-1 down and he finally threw in the towel after losing the first set 6-1.
"I had some pain in the knee yesterday. I felt the pain a lot when I woke up this morning," Nadal told a news conference. "When I push (on my knee) I feel a sharp pain."
Asked if he was worried about his fitness for the season-ending Masters Cup in Shanghai which starts on Nov. 9, he added: "For sure. Hopefully it will be fine ... (but) I don't know".
Nadal, who said he had never had this kind of injury before, is due to fly to China on Tuesday but may now have to change his plans.
"With this pain maybe (I will go) a little later. I don't know, it depends," said Nadal, who could not bend his knee properly.
The Spaniard hit out at the hectic tennis calendar.
"I think with this calendar it's very difficult to play a lot of years in a row," said Nadal. "I think the ATP and everybody have to think about these things happening at the end of the season."
Asked whether he knew world number two Roger Federer had also pulled out of his quarter-final against American James Blake, Nadal said: "I saw him in the locker room five minutes before my match and he told me he had a pain in the back.
"I said, maybe we are both going to be going home tonight."