Rafa Nadal has been forced to pull out of his semi-final againstAndy Murray at the Sony Ericsson Open due to a knee injury, the world number two said on Friday.

"I am very sorry for my fans, those that are here in Miami and those around the world," Spaniard Nadal said on his Twitter feed. "I do not feel able to play today because of my knee."

Briton Murray, the fourth seed and 2009 champion, will play the winner of the other semi-final between top-seeded Serb Novak Djokovic and Juan Monaco of Argentinawhich takes place later on Friday.

Nadal, who has suffered from tendinitis in his knees several times during his career, had dropped only one set on his way to the last four as he chased a first title since last year's French Open.

The 25-year-old, runner-up in Miami three times, has won 13 of his 18 meetings with Murray, although all five of the Scot's victories came on hard courts.