World number two Rafael Nadal could join the list of high-profile withdrawals from the Madrid Masters Series if the tendinitis in his knees does not improve before his opening match on Wednesday.

The 19-year-old top seed withdrew from the ATP event in Vienna last week. Although he said he is determined to play the Madrid tournament, the pain in his knees could cause him to drop out again.

"I've been training for a couple of days and the knee is still hurting," said Nadal. "I've had a scan done and it shows that I've got acute tendinitis."

Coach Toni Nadal said that a decision would be taken after consultation with the player's doctor.

The two million euro ($2.4 million) hardcourt tournament has been hit by the withdrawal of some of the biggest attractions on the circuit. World number one Roger Federer, Lleyton Hewitt, Andre Agassi and last year's winner Marat Safin have all pulled out because of injury.

Nadal has been given a bye into the second round and is due to play Victor Hanescu of Romania on Wednesday.