Eh, he's only gonna play one singles match and probably the doubles match. That hardly counts as tiring. Don't tell me Sijsling can actually take a set from him, to keep Fed in for more than 2 hours.
used to think the same but the prep they put in for DC is to practise hard all week on clay and then play best of 5 matches three days in a row - its really like an extra tournament added
it does more damage than you think
the quick surface change also irritates