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.
it's not sure it's Sijsling, actually it'd rather be De Bakker who's recently won two challengers on clay, one last sunday, who is not back to his best level but still is back at a good level on clay and has often played wll in Davis cup (defeated Monfils on clay).
Sijsling (who was the title holder) was out of last week challenger for whatever reason.
Anyway, Wawrinka often has problems against Haase (he's 5-0 in the H2H but all but one were tight matches, including on clay) and the dutch doubles team is good, then it's far from sure that Roger only has two matches to play : most of the time he has two singles to play when he plays Davis cup.