World number one Rafael Nadal is concerned that a foot injury may force him to quit Wimbledon.The defending champion produced a rousing performance to see off Juan Martin Del Potro in four sets to move into the quarter-finals.But the 25-year-old Spaniard was troubled by the injury throughout and will go for an MRI scan on Tuesday."I don't know what the problem is, I will get it checked out. I'm worried for sure," he said."At one moment at the end of the first set I thought I might have to retire."To hit my forehand was very tough. I felt something really strange when it was at 6-5 in the first set when I called to the trainer. I pushed hard with a forehand and I just felt something crush in the back of the foot. I didn't know what went on."I changed the position of the foot so I was able to continue but with pain."It is not the first time injury problems have affected Nadal at Wimbledon - he was unable to defend his 2008 title because of knee problems - and the Mallorcan is concerned about what his scan may reveal.He will play Mardy Fish in the quarter-finals on Wednesday if he is fit enough and then could play Andy Murray in the semi-finals if the Scot beats Feliciano Lopez."I'm going to do the MRI. We'll see what's going on. I cannot predict the future," Nadal said."I am checking with the doctors, with the physios. They are trying. We are trying to do our best as fast as possible."Let's see what's going on and let's see how the MRI looks. And after, let's see if we have the chance to recover for Wednesday. I don't know."The champion had 10 minutes of treatment at the end of the first set and thinks the strapping which his trainer put on his left foot enabled him to continue."With the tape it changed the direction of the support and it didn't hurt me that much," he said.Del Potro had his own injury scare, slipping and falling to the grass at 2-2 in the third set.He remained on the ground, clutching his left hip and then left the court for treatment but returned within minutes to continue as Nadal won 7-6 (8-6) 3-6 7-6 (7-4) 6-4.Source: BBC