The Spaniard was bothered by the problem in his three-set quarter-final win over Jo-Wilfried Tsonga and has decided not to take any risks with his long-term fitess.

It is the second time this week that Murray has benefited from a walkover after Milos Raonic withdrew from their third round match.

The news means the world number four will now face Novak Djokovic after he defeated 21st seed Juan Monaco in the other semi-final.

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

The Spaniard was playing in just his fourth event of the season and he has not won a title since last year's French Open in Paris.

"We are disappointed that Rafa has been forced to withdraw from the tournament but the health of the players always needs to be the number one priority," said Adam Barrett, the tournament director.

"He has played some great tennis and is a tremendous ambassador for the game. We wish him a speedy recovery and hope to see him back on the court soon."