By Simon Evans

MIAMI (Reuters) - Rafa Nadal was forced to pull out of his semi-final against Andy Murray at the Sony Ericsson Open because of a recurrence of a knee injury.

Nadal, who has had problems with his knees before on the American hardcourts, apologised for withdrawing from the Masters series event, but said he had to.

Murray, the Miami champion in 2009, was gifted his second walkover of the tournament after Canadian Milos Raonic pulled out of their third round match with an ankle injury.

The Scotsman will play the winner of the other semi-final, between top-seeded Serb Novak Djokovic and Juan Monaco of Argentina, which was due to take place later on Friday, in Sunday's final.

"To get two in one week is strange. That doesn't happen often really at all," said the fourth seed.

"It's never happened to me before. So I don't really know how I'm gonna feel for the final, but I'll definitely be fresh."

He suggested the injury was probably a recurrence of his previous knee troubles.

"I have to see a doctor but it looks like nothing really, different than what happened a few times in the past," he said.

In 2009, Nadal pulled out of Wimbledon, where he had won the year before, because of knee problems.

At the 2010 Australian Open, where he was the defending champion, he retired from his quarter-final clash with Murray, again because of knee problems.

But the 25-year-old Spaniard said his knees were in better overall condition than they had been and he was confident of being ready for the upcoming European claycourt season.

"The tendons are much better today than three years ago. The treatments worked fantastic," he said.

"Even if today I have a really bad knee and last couple of days were tough for me, the positive thing is the tendon has improved a lot in the last couple of years.

"This year I started well with no problems. Today is bad news but that's the sport. We cannot expect, playing as much as we play, to be perfect every day of our life.

(Editing by Julian Linden)