Don't doubt just yet. I think history shows, except for those couple years where Nadal simply was an unstoppable juggernaut in Paris*, that he has played some shaky tennis in the first week of grand slams throughout his career on all surfaces.
It seems silly to even say this, but the best chance to take Nadal out at these slams is to get him in the first week. Nadal plays tense and defensive until he gets into his groove during the second week. And by the time he hits his groove, it's too late to touch him. The problem is that his opponents during the first week of slams are scared xxxxless to play him and he gets the wins.
I think the sets he has lost so far this week are representative of that trend. Sometimes those early rounds guys show up with a nothing-to-lose attitude and might take a set off of Nadal. To be fair to Brands, had he won that second set tie-breaker in the first round, he just might've knocked Nadal out a la Buster Douglas v. Tyson.
Doesn't matter cos Nadal will be in full form when he meets Djokovic in the semi-final. Heavy, damp conditions favor Djokovic. If the sun pops out and dries things off, then I see Nadal in four close sets (maybe a break in each set and/or a couple tie break sets).
Only after we see the carnage from that match will we know what the final has in store regardless of the player that makes it through in the bottom half of the draw.