From my experience and understanding, the strings and tension make most of the difference, frames are generally always constructed solid. Of course we know most of the pro's have customised racquets, although companies like Wilson release new racquets every so often, the ones they give the pro's are just paint jobs for marketing reasons. Wilson must know exactly the racquet Del Potro uses, it will be in their 'secret' directory along with all the others. Federer has been using the same racquet for like 10 years, Wilson provided exactly what he needed. Federer would not want to change his game to match a new racquet, and this applies to all the Pro's who regularly receive new racquets. Sure this is all in Del Potro's mind? Otherwise every time a pro received a racquet their game would have suffered? The man has a excellent forehand, hard and low, and efficient backhand, he is one of the best base liners and very good mover for his size. Using a 5 year old frame and coming to 5 set matches with 2 racquets, surely he is not doing himself favours. Cmon delpo sort it out.