Rafael Nadal withdrew from the Miami Masters tennis event before his scheduled semi-final against Andy Murray on Friday with a knee injury.

“So sorry for my fans, those here at Miami and those around the world,” the Spanish world No.2 wrote on his Twitter account. “My knee won’t let me play today.”

It’s a blow for Nadal with the start of the claycourt season looming.

Nadal is playing in only his third tournament of 2012 having fallen in the semi-finals in Doha, losing to Novak Djokovic in January’s Australian Open final and making the Indian Wells Masters semi-finals last week.

His last title came at the French Open last year.

“Rafael Nadal has been forced to withdraw from today’s match at the SEOpen due to a knee injury,” a posting on the ATP’s Twitter feed said.

“Andy Murray advances to the final.”

Later on Friday, world No.1 Djokovic was to face Juan Monaco for a place in the final.