After watching the USO final again, its obvious Fed's still fighting the racquet. I think the racquet is a too powerful for him. Most of his shots had way more pace than with the 90, but they also had a lot more spin. When he needed to straighten out the shot he struggled. There's no doubt Fed made more errors in the USO final than in previous finals (especially with the 90), and IMO the racquet was a major contributor. He needs something slightly bigger than the 90, but less than the RF97....
Feds Racquet
Feds Racquet