Hopefully he was running on fumes, although I doubt it.
The times are very strange, 1.5 between Vettel and Hamilton, about 1 second between Hamilton and Alonso.
From Andrew Benson - BBC "Sebastian Vettel rather made jaws drop with his lap at the end of first practice - 1.4secs quicker than Lewis Hamilton in second place. But the lap was not as impressive as it at first appears - the Red Bull was on effectively a new set of tyres; they had done only an installation lap. McLaren, for one team, seemed unperturbed."