Nadal said it was a recurrence of the problems that forced him out of the defence of his Wimbledon title and away from the ATP Tour last year for more than two months.
There must be doubts as to whether the 23-year-old can scale the heights he did 12 months ago when he held the Australian, French and Wimbledon titles and was ranked number one in the world.
"Don't start guys with these questions right now," Nadal told a news conference. "I think it is not the right moment.
“It is impossible for me to know after half an hour."
"For me it's a little bit disappointing (to) have the pain on the knee because I think I am very close to start winning these important matches," he told reporters.
Published 26 January 2010, 17:13 IST