Fernando Alonso says the closing laps of the United States Grand Prix were a struggle for him due to "huge vibrations" from his tyres.

Sebastian Vettel closed to within half a second of Alonso on the final lap having made a late stop for soft tyres. While Alonso himself had also had a third stop to take on softs, he said a problem with his tyres had made visibility a big problem and he had to take risks on the final lap to ensure he kept Vettel at bay.

"I had huge vibrations in the tyres for the last two laps, so I could not even see the track basically," Alonso said. "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 [Kevin] 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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While Alonso says he feels he made the best of a tough weekend, he urged Ferrari to push in the final two races as he is wary of McLaren becoming a threat in the constructors' championship.

"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 need to control the pace a little bit and then secure this sixth place that was important.

"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."