i read his autobio too..... very disappointed as there's so little actual tennis in it. he talks mostly about scores. or i came back or he came back.. never once talks about his serve or his volley. talks once about diving (correct me if you disagree).
seems like 3 things may have contributed to his later years.
tormented individuals. hadn't won a title in 5 years when he had his tax trial. seemed to weigh on him alot.
seemed to stay at baseline more later in career, where he wasn't better than the others.
his serve was revolutionary (i think) when he came on scene. guys adjusted to it somewhat (see roddick too)... and did they change the balls on him?
boris' autoB was pretty big letdown. just talks about how hard he has it. throws in how much he admires ali right before he mentions impregnating a woman he'd met 5 minutes earlier when his wife was in hospital potential with a premature delivery. great that you respect ali so much!