Red Bull team principal Christian Horner has admitted that Mark Webber's retirement from the United States Grand Prix due to alternator problems has concerned him.

Webber retired early on in the penultimate race of the season which saw Sebastian Vettel extend his drivers' standings lead over Fernando Alonso to 13 points with just one race remaining.

However, after Red Bull's third alternator failure of the season, Horner is worried that the problem could dent Vettel's Formula 1 Championship dreams in Interlagos on Sunday.

Horner told reporters: "It's just a worry, full stop.

"Unfortunately it is the third alternator failure we've had and there have been others in other cars.

"The new version has raced on other engine cycles and hopefully that's what we'll have for Brazil."

Vettel will win his third Formula 1 title if he finishes in the top four in Brazil.