Well, Stan didn't seem injured and had no known reason to be tired, so I'll take the drunk/hungover option
We also forget that Stan is now an old man at 30 (older than Nadal), so expecting day in day out consistency which he didn't have in his younger days is probably too much to ask for.
After the Fed-Djoker match , there should be more clarity on how things will pan out (since we will know who tops that group). If we assume Djoker tops and Fed is second, Murray-Nadal match will be the tiebreaker for who avoids Djoker in the semi.