I don't think it's necessarily grass. If it were a choice between the 4 slams, then yes, Wimbledon would be the one he's been underperforming at the most. But if there were more tournies on grass, I don't think he would overall perform the worst there. For example, I think Fed has better chances against Nadal on grass, than on slow HC, for example.
I think it's close between grass and slow HC.
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