Nadal missed two months with knee tendonitis last year
World number two Rafael Nadal will miss up to four weeks as he recovers from a knee injury.
The 23-year-old retired from his Australian Open quarter-final against Andy Murray while trailing 6-3 7-6 3-0.
The six-time Grand Slam champion has been diagnosed with a small tear in the back of his right knee.
The injury, which Nadal has described as a "little tweak", is not thought to be related to the knee tendonitis that blighted his 2009 season.
The Spaniard will miss the ATP tournament in Rotterdam, where he was beaten by Murray in last year's final.
He could return at the Indian Wells Masters in mid-March.
His world number two ranking is set to come under threat from Murray and Novak Djokovic, but Nadal said: "It is not important for me the ranking issue and losing a few spots, that's normal when you are not able to compete.
"For me it is more important to play well again and to have the good feelings on the court that I had in Abu Dhabi, Doha and also at the Australian Open where I played at the highest level."
Dr Angel Ruiz-Cotorro, who examined Nadal's injury, said the four-time French Open winner would have to rest for two weeks before he "steadily resumes his sporting activity".