F1 USGP: Low-downforce set-up hurt Sebastian Vettel in traffic

The German had opted for a set-up aimed at top speed in a bid to boost his overtaking chances, as he was forced to start from the pit lane because of a power unit change.

But despite the obvious speed of the Red Bull, Vettel failed to make much progress early on and complained on the radio about how slow was.

Over the second half of the race matters improved, as he charged through to seventh, which left Red Bull convinced that the problem was related to difficulties caused by battling other cars.

Team boss Christian Horner said: "It was a race of two halves for Seb.

"The first half was very uncompetitive and he was struggling to match the times of the Saubers and Force Indias but thereafter it came alive for him."

"We went for a very low downforce set-up with Seb that he was working on through practice.

"But I think when he was in dirty air, he just could not generate the grip. It had quite a detrimental effect on the car.

"It was only once he got, after the stop, on to fresher tyres, the balance seemed to come alive for him.

TACTICAL DOUBLE PIT STOP

Horner also explained that the double pit-stop for Vettel under the safety car, when he ran the soft tyres for just one lap, was done for tactical reasons.

"We stopped because we didn't intend to use the soft tyre again," he explained. "We effectively were going to do the race in two halves.

"So we cycled the option tyre because he didn't give any ground away behind the safety car.

"We got that tyre out of the way without conceding any ground, but then the degradation on the harder tyre was high and there was a risk to get to the end.

"So that's why we decided with eight laps to go to put a set of soft tyres on and get back to where he had been. It was a safer way to get to the end of the race."