That's what I am wondering.
Is it career-wise (ie. their peak), or their current condition?
Hm.. I took a different approach where I tried to base it on their performances over the past couple of years. For most players I probably emphasized this one, but for Nadal I tried to rewind the clock a bit to compensate for his injuries.
Even still, it's not a stretch to place him in the top three for slower hard courts if one were to judge him on this year's AO.