his game is anchored on defense based around his unyielding stamina, which is very taxing on the body and knees so it is much more effective on clay than off of it. he was able to win everywhere when he was in his prime, but he turned pro very young and has played for a long time, his tennis years only slightly less than federer so even though he is 4 years younger than fed his body is declining already and deteriorating physical abilities means he is much less effective at playing his game off clay. also last year djokovic gave him no chance, and this year it seems like it seemed like the trend of slowing down courts at all tournaments has reversed ever so slightly.
Perfect reasoning. However why does he continue to win on clay where maximum physical effort is required?