I just read an article about Jo complaining about the unprofessionalism of the ball kids. He obviously said that davydenko played well and although he was younger, Davydenko's physical conditioning is better than him. He said that the ball kids didn't work well with him. When he need to serve, they didn't pass him the balls at the right place. When he needed to wipe his face, they couldn't get him the towel. He said that was a setback and waste of his energy. In the last set alone, the ball kid lost sight of his towel like ten times and needed time to find it. It was a distraction to his concentration.
I believe what Jo said. I didn't see this match but I saw the match earlier and del potro was mad that he had to fetch his own towel many times, like the kids either didn't know the word towel or they were deaf.
edit: I found the video to his post match press conference