Originally Posted by degrease
As a racket gets older the graphite or whatever "softens" giving it a different feel. James blake also had this problem. I think i read del potro using an old k factor but giving him new ones dont feel the same. Pro's have very sensitive feel so minimal changes they can register
Del Potro never used a K-Factor. It's a paintjob. He uses the old ProStaff 6.1 Classic underneath the paintjob. A racquet which came out in 1991. Wilson hasn't made them in a very, very long time, which is probably why Del Potro can't get any more.