Sounds like Federer had trouble seeing in the match vs. Nadal at the French in 2005, or so the commentators said. It was dusk and the powers that be weren't turning the lights on for whatever reason.
As far as other matches go that he loses, he probably doesn't go "all out" at EVERY match because he'd burn out/acquire injuries. Sounds like players have to be superhuman to deal with this modern tournament schedule anyway.
Anyway, Haas is Fed's man to contend with...