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Fernando Alonso says Ferrari now has to be wary of McLaren in the constructors' championship after losing more ground to Williams at the US Grand Prix.

After scoring just eight points to Williams' 22 in Austin, Ferrari is now 42 points off third place in the constructors' championship and 49 ahead of McLaren in fifth. Alonso, who is not expected to be at the team next year and has been linked to McLaren, said Ferrari's focus now has to be on holding fourth rather than regaining third.

"We were struggling over the weekend but we were also doing that in Russia and a little bit in Japan as well. We just need to keep scoring points because the constructors' championship is still there and we have some fights. We thought we had lost too much with Williams and we will be in the middle of nowhere for the remaining races, but now McLaren seems to pick up the pace so we cannot afford to not score points. We are still in the fight."

Alonso said he could not have hoped for much better than his sixth place finish on Sunday.

"It was a race as we expected; not easy, not competitive as we were not competitive over the rest of the weekend. We did a good start, but after that we could not follow the first five cars and they disappeared very quickly. We just tried to control the people at the back, they were doing some different strategies after stopping on lap one when the safety car came out. We just needed to control the pace a little bit and then secure this sixth place that was important."

At the end of the race Vettel closed in and Alonso said a tyre vibration meant he could barely see where he was going.

"I had huge vibrations in the tyres for the last two laps, so I could not even see the track basically. I tried to control the pace and the gap to the cars behind to try to cross the line just in front of them in a safe manner. But then Vettel overtook Magnussen and there was no time to save any more and on the last lap I was flat out with huge vibrations."

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