Vettel must now win all three remaining races in Mexico, Brazil and Abu Dhabi to have any chance of beating Mercedes rival Lewis Hamilton, and that will still not be enough on its own.
Sebastian Vettel would have to make the greatest comeback in Formula One history to win his fifth championship this year and even the Ferrari driver is not buying that one.
The German must now win all three remaining races in Mexico, Brazil and Abu Dhabi to have any chance of beating Mercedes rival Lewis Hamilton, and that will still not be enough on its own.
Hamilton, the winner of nine races this year to the German's five, has a 70 point advantage after Sunday's US Grand Prix and needs to score only five more to become a five-time champion.
He will not even need those if Vettel falls short of perfection.
British bookmaker William Hill was offering Hamilton at 1-500 with Vettel 50-1 -- the German on the same odds as Hollywood actors Leonardo DiCaprio or George Clooney becoming the next US president.
Realistically, it is all over and the coronation is likely to come in Mexico City next weekend.
The last time Hamilton went three races in a row without scoring at least five points was in 2012, when he was at McLaren and failed to finish in Valencia, was eighth in Britain and retired in Germany.
It has never happened in his time at Mercedes, and Hamilton has not had three races in a row off the podium in a single season since 2013.
Vettel has not won three races in a row since he was at Red Bull in 2013.
Ferrari last won in Mexico in 1990 -- admittedly not as dramatic as it sounds because that race was not held from 1993 to 2014 -- and have never triumphed in Abu Dhabi in that grand prix's nine-year existence.
Asked about his championship chances after starting fifth in Austin and dropping to 15th following a first lap spin before he eventually finished fourth, Vettel smiled.
"Getting slimmer," the German, whose teammate Kimi Raikkonen won the race with Hamilton third, replied.
The grim humour continued when it was pointed out to him that he had to win the last three races: "Yeah, probably the last four or five," he said.
"So far we didn’t do so well. Obviously, the last couple of months haven’t been very good for us in many regards.
"I had another recovery drive (on Sunday) but I’m getting a bit tired of recovery drives."
On the plus side, Austin showed that Ferrari had at least regained some of their speed to end Hamilton's bid for a fifth win in a row.
That, however, was largely due to discarding some recent changes and reverting to what worked before.
"You can see it as good news but you can also see it as bad news. If we have to go back to a car that has been competitive three or four months ago, then surely it can’t be good news. If you think about it," said Vettel.