Michael Schumacher took five years to win a Formula One world title with Ferrari, and Sebastian Vettel is highly unlikely to do it in four years.

Vettel travels to Sunday's United States Grand Prix 67 points behind Mercedes' Lewis Hamilton, allowing the Briton to clinch a fifth career title if he garners eight points more than his rival, such as winning and Vettel failing to come second.

But Vettel insists he is not throwing in the towel, stating "the spirit is unbroken despite everything" after having to settle for sixth at the last race in Japan.

"We know it is difficult from where we are in the point standings, but we don’t have much to lose," he said. "So we keep fighting and resisting and we’ll see what the other races bring."

Like Hamilton, Vettel currently has four world titles, but they are all from his term at Red Bull, won 2010-2013.

The German joined the Scuderia in 2015, full of praise of Ferrari's myth and determined to end their drivers' championship rot since Kimi Raikkonen's success in 2007.

"There are so many fairy tales on Ferrari and what it feels like driving a red car. I the end I can only confirm these fairy tales. It is not just a story, a real legend exists," Vettel said.

The statement came after the first test laps but the title dream remains unfulfilled as Vettel had to come to terms with the harsh realities.

Ferrari were not able to compete with Mercedes in his first two seasons, and 2018 seems a repeat from last year when Vettel also led early in the season but Hamilton then coasted to the title.

Strategy errors and personal mistakes have cost Vettel especially in the second half of the season where Hamilton has been imperious by winning six of the last seven races.

Vettel has been criticised for his driving but he has also had support, including from Hamilton who last week asked the media to show "a little more respect" for Vettel.

"You simply cannot imagine how hard it is to do what we do at our level, for any athlete at the top of their game that is. It is to be expected that being humans we will make mistakes but it is how we get through them that counts," Hamilton said.

Vettel could take heart from the fact that the iconic Schumacher needed five years at Ferrari to win the first of five titles between 2000 and 2004, after arriving in 1996 as a two-time champion.

But he doesn't like the comparison too much, and instead it was the beleaguered team principal Maurizio Arrivabene who used it before the departure to Austin.

"I am convinced that Sebastian Vettel will sooner or later win with Ferrari," Arrivabene said.

"It takes time and determination, without it you achieve nothing. Michael Schumacher was like that, and Sebastian Vettel is very similar, his four world titles with Red Bull prove it."