Yep I think he is tired physically and mentally. And when that happens during a match everything goes. He probably couldn't do his pre season training block to his usual intensity and maybe he is suffering now because of it.
However IMO if a tennis player steps out onto the court he is fit to play. No question. No debate. So Andy is fit and I personally hope we have heard the last of posters on here using his back/surgery/weather whatever as an excuse. Bottom line is its disrespectful to the opponent.
I don't think that's the case, BN. Elite athletes carry knocks and aches and pain pretty much all of the time, due to the extreme pressures they put on their bodies.
I'd recommend reading Agassi's autobiography, where he describes the constant battle against pain and injury he encountered.