MIAMI (Reuters) - Rafa Nadal has been forced to pull out of his semi-final against Andy 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."

(Writing by Iain Rogers in Madrid, editing by Ed Osmond)