World number two Rafael Nadal [ Images ] 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 [ Images ] 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 [ Images ], Lleyton Hewitt [ Images ], Andre Agassi [ Images ] and last year's winner Marat Safin [ Images ] 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.