Rafael Nadal has a partially torn patella tendon in his left knee and will be sidelined for at least the next two months, including Spain's upcoming Davis Cup semifinal against the United States.
"I have to recover and I'll be back once the pain is gone," Nadal said in a website posting linked to his Twitter feed.
The 11-time Grand Slam champion has been out of action since losing in the second round of Wimbledon in late June.
"I have missed the Olympics and the U.S. Open in the last few weeks, two of the most important tournaments of the year and that I really wanted to play," Nadal said. "I really want to be back competing and enjoying the tennis tour, but I have many years in front of me and my knee needs some rest."
Originally, doctors told the Spaniard he had tendinitis in his left knee and needed to rest 15 days.
Nadal's statement said since the tear was diagnosed he has been receiving treatment that includes medication, physical therapy and a rehabilitation program.
"I feel better after the meeting with my doctors and happy knowing that the evolution of the past weeks has been positive and surgery has been avoided," he said.
This isn't the first time Nadal's knees have forced him to take a break. In 2009, he took a hiatus, recovered and came back stronger than ever.
The season-ending ATP World Tour finals begin in London on Nov. 5.