Luck does have something to do with it, but he's been riding smarter because he's better placed in the peloton (his team has done an excellent job protecting him). Contador doesn't seem to be focused at all (not like recent years), and his team looks worn out too.
Andy doesn't have to drop Contador, he can ride defensively. I do think he will try to take advantage, as dlwssonic said, of his fatigue and knee pain. He will want a cushion in the TT, but he doesn't need to actually drop Contador, not with his time advantage.
If Contador can only take 39 seconds from Andy on a 52km TT, he will take less on a 42.5km TT. If anything Contador has to drop Andy. I hope Andy's mechanic installed a chain watcher
And they both have to watch Evans...