Shanghai - World number one tennis player Rafael Nadal is suffering from serious physical problems, the president of the International Tennis Federation (ITF), Francesco Ricci Bitti, said on Wednesday.

"Nadal has a very serious problem," Ricci Bitti told a small group of media, including Deutsche Presse-Agentur dpa, at the Masters Cup tournament in Shanghai.

The Spanish world number one declined to take part in that season- ending event, and eventually also stepped down from the Davis Cup final that Spain is set to play away against Argentina November 21- 23. He is suffering from knee tendinitis.

"I already saw him very tired at the (Olympic) Games, particularly after the great Olympic semifinal (against Novak Djokovic), one of the best matches this year. Nadal won because he is a great player and a great fighter. Nadal played the final at 70 per cent (of his level), but that was enough," Ricci Bitti said of the Spaniard's gold medal.

The ITF president noted that he is a friend of Nadal's and that they even share the medical services of Spanish doctor Angel Ruiz Cotorro.

"This year he has delivered a lot, he has delivered too much," Ricci Bitti said of the player.

Still, the Italian noted that the Davis Cup final in the Argentine resort of Mar del Plata will not suffer the top player's absence too much.

"It would have been better if he were there, but it's no drama. The Davis Cup is very strong," Ricci Bitti said. "The Davis Cup competition is stronger than players, although Nadal is the number one, he is very important."

"I am sorry because he is my friend, a great player and a great person who contributes very generously to the promotion of our sport," he added.

In 2008, Nadal reached the world number one spot previously held by Roger Federer, and won his fourth consecutive French Open and his first Wimbledon as well as the Olympic gold medal.

However, he is said to require a further break of three to six to get back to his best level. The first 2009 highlight is the Australian Open Grand Slam which starts in mid-January.

In Nadal's absence, an Argentine side led by David Nalbandian and in-form youngster Juan Martin del Potro is the favourite to lift its first "salad bowl," as the Davis Cup trophy is known.

"Now Argentina is the favourite, that is clear," Ricci Bitti said. (dpa)