World number three Rafael Nadal has pulled out of the Davis Cup semi-finals with a knee injury and will stay on the injured list for the next two months, he said in a statement on Monday. \"I really want to be back competing and enjoying the tennis tour but I have many years in front of me and my knee needs some rest,\" the 26-year-old said. \"I will be back when I have no pain and able to play fully fit.\"