Rafael Nadal has tendinitis in his right knee and could be in doubt for Spain's Davis Cup final against Argentina.

Medical tests on Tuesday confirmed the tendinitis, and Spain team doctor Angel Ruiz-Cotorro said the top-ranked player would undergo treatment through Saturday. Nadal will have further tests on Monday to assess the injury.

Spain plays Argentina in the final on indoor hard court at Mar del Plata from November 21-23. Spanish Davis Cup captain Emilio Sanchez Vicario will not pick Nadal if he is unfit, Nadal's uncle and coach Toni said on Tuesday.

On Mobday Nadal pulled out of the season-ending Masters Cup in Shanghai citing fatigue after a long season in which he's played in 111 matches.

Nadal, winner of a fourth straight French Open title and first Wimbledon championship this year, withdrew from the Paris Masters last Friday with a knee problem after losing the first set of his quarterfinal match against Nikolai Davydenko.

Nadal won two Grand Slam titles and the Olympic gold medal this year as he supplanted Roger Federer at No 1. Without Nadal, Spain would be regarded as a big underdog to lift its third Davis Cup trophy in eight years.

Spain beat the United States 4-1 in the semifinals, with Nadal and David Ferrer playing singles and Feliciano Lopez and Fernando Verdasco the doubles.