Rafael Nadal has won two Grand Slams this season
World number one Rafael Nadal has pulled out of the season-ending Masters Cup in China due to fatigue.
"I have decided not to compete at the Masters Cup in Shanghai," said a statement on Nadal's website.
Last week the 22-year-old Spaniard retired during his Paris Masters quarter-final match with Nikolay Davydenko due to a knee injury.
Five days after the eight-man tournament in Shanghai ends, Spain will face Argentina in the Davis Cup final.
France's Gilles Simon will take Nadal's place in the season-ending tournament, which starts on Sunday.
Roger Federer, Novak Djokovic and Britain's Andy Murray are now the leading contenders.
World number two Federer has been struggling with a back injury, but hopes to make the event.
Nadal said: "It has been a long and difficult year where I managed to obtain great results.
"I have mentioned on various occasions that the tennis calendar has been extremely hard with practically every single week playing.
"This forces us players to put pressure on our bodies, making it impossible for a top level player to be 100% at each event.
"On a personal level, I had as one of my goals to become number one during this year.
"(But) competing at so many events might have harmed, especially at the end of the season, my physical condition, taking away the freshness needed to play at a high level in these last events.
"I don't know if this has been a mistake or not but the fact is that with the goal achieved I also have to take one of the most difficult and painful decisions.
"I have decided not to compete at the Masters Cup in Shanghai.
"I know that many people were speculating about this matter and even though I have yet not done any tests, I can honestly say I have taken the right decision.
"I want to recover and get ready for the Davis Cup final in Mar del Plata, Argentina. That is also another reason for not going to Shanghai."
Nadal ends 2008 as world number one after claiming the Wimbledon and French Open Grand Slam titles.