Sebastian Vettel salvaged a seventh-place finish after starting the United States Grand Prix from the pit-lane, but it wasn't easy going for the world champion.

Needing a new engine package ahead of the trip to Austin, Vettel was obliged to start from the pits, and effectively sacrificed his weekend to race preparation after dubbing qualifying 'pointless'. He had nevertheless headed into the weekend expecting to be able to pocket a decent score, but the early stages of the race proved to be tough sledding for the German, who made two pit-stops in the first three laps behind the safety car in a bid to open up his strategic options by ridding himself of the mandatory run on the softer Pirelli.

Even without the absent Marussia and Caterham cars, Vettel struggled to break into the top 15 once the race restarted, complaining of a lack of grip that left him vulnerable to attack from the likes of Romain Grosjean.

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A mid-race stop then appeared to transform the German's fortunes, allowing him to make up ground and latch onto the battle for the minor points positions as Jenson Button's McLaren held up a squabbling pack of cars. Another stop, fitting the softer tyre late in the day, then propelled Vettel into a pursuit of Fernando Alonso's Ferrari, which ultimately came up half a second short of netting sixth place.

"The first half of the race wasn't so good," he confirmed, "In the second half, it came more alive and we were able to make up some ground with a pit-stop near the end of the race.

"The low downforce set up was good for passing, but it made it difficult to stay close to people. We decided to go for fresh tyres with only a few laps to go, so we were in a stronger position at that point compared to others who were struggling on older tyres and the pace was better."

Vettel's poor 'first' stint raised questions about the performance of his new engine, but team principal Christian Horner reported that the Austin breeze had played a part in disrupting the champion's race, and Renault's Thierry Salvi insists that there was no issue with the unit.

"He struggled a lot with the car during his first stint, but then came alive as the fuel load dropped," Salvi revealed, "There were no problems with his Power Unit, which helped him fight his way back up to seventh on the option tyre at the end. It's now a question of being able to complete the final two races with these Power Units. Reliability is obviously key and we'll be looking at everything in order to preserve them."