World tennis No. 1 Rafael Nadal pulled out of next week's Tennis Masters Cup in Shanghai because of a knee injury and will focus on Spain's Davis Cup final against Argentina later this month.

Nadal, who has played 93 singles matches this year in his quest to become the world's top player, said his workload "might have harmed my physical condition, especially at the end of the season." The decision not to compete in Shanghai had been "one of the most difficult" of his career, the French Open and Wimbledon champion said on his Web site.

Nadal last week withdrew from the BNP Paribas Masters quarterfinals in Paris with the injury to his right knee.

The 22-year-old, who ended Roger Federer's record 237-week reign at the top of the rankings in August, said this year's tour schedule had been "extremely hard," making it "impossible" to compete 100 percent at every event.

The Spaniard's place in Shanghai, where the top eight men will be competing Nov. 9 through 16, will be taken by France's Gilles Simon.

The Davis Cup final takes place Nov. 21-23 in Mar del Plata, Argentina.