Formula One has had to modify its qualifying format after financial problems ruled the Caterham and Marussia teams out of Sunday's US Grand Prix while Red Bull's Sebastian Vettel is set to be sidelined by an engine penalty.

Stewards said in a statement that the slowest four cars in the first session would play no further part in qualifying with the next slowest four also dropping out in the second phase.

That will still leave 10 drivers fighting for pole position in the final phase.

Previously, the slowest six dropped out in each of the first two sessions with the Caterham and Marussia cars usually in the initial group.

Caterham and Marussia are both in administration and seeking new ownership to survive.

With just nine teams now competing, Formula One will have fewer cars in qualifying than at any time since 2005.

Tomorrow's session is likely to see only 17 cars on track, with four times world champion Vettel facing a pit lane start because he is set to exceed his season's allocation of five power units.

"As far as I know, the plan is to use the opportunity to change, to have a sixth component of all the various components," the German told reporters on Thursday.

"Obviously for that as far as the regulations are concerned we have to start from the pit lane."

Without having to think about qualifying, Vettel can use practice to focus entirely on the race setup.

"If he qualifies on pole he still has to start from the pit lane... why would you put miles on the engine?," said Australian team mate Daniel Ricciardo.

"It's not ideal for him and for everyone, for the fans...you want to be out there, you want to drive. You want to give the fans what they came for."