I think it's utterly impossible to determine in any quantitative fashion whether the changes in results of a single player are due to aging or to increased competition. Something along the lines of the pro-am split before the Open Era might have made that determination a lot easier, but these days it seems circular. Are Tsonga, Berdych, and Del Potro better now because they beat Federer more often? Or is he simply worse as a player? The only constant in Federer's career has been Nadal - he's always struggled with him. He had a highly superior record against Djokovic until 2011 - the year he turned 30 and also the year Djokovic played better than he ever had or has since. He didn't lose to Murray in a slam until he was 31, so is Murray tougher competition than Nalbandian who beat him twice in slams in 2003 alone? There's just no numbers to look at, unless you look at the kinds of numbers ambk is, which are still very much open to interpretation.