Devastated Rafael Nadal vowed the knee injury that forced him out of the US Open semi-finals on Friday won't stop him chasing more Grand Slam glory.

The Spaniard, his world number one ranking still safe, said the trouble was the familiar tendinitis he's dealt with for a decade -- unpredictable but not career-threatening. "I know what I have," he said after limping off Arthur Ashe Stadium having lost two sets to third-seeded Argentine Juan Martin del Potro. "I know what is going on with the knee. I know how I have to work to be better as soon as possible."