Vettel need finish only fourth in the Sao Paulo finale to take the title but the weather at Interlagos can be fickle and the wily and consistent Alonso cannot be ruled out for what would also be his third title.
"The race unfortunately hinged on one backmarker who didn't use his mirrors, that was the difference," said Horner after Hamilton passed Vettel for the lead on lap 42 after the German had lost time behind Indian Narain Karthikeyan's HRT.
"There was nothing between the two of them today. They were trading times all afternoon and it was that one backmarker that cost us.
"But congratulations to Lewis, he's obviously had bad luck at other races and it was important to finish ahead of Fernando," said Horner.
Image: McLaren Mercedes driver Lewis Hamilton, middle, of Britain, Red Bull driver Sebastian Vettel, left, of Germany, and Ferrari driver Fernando Alonso, of Spain, drink champaign on victory podium after the Formula One U.S. Grand Prix auto race at the Circuit of the Americas Sunday, Nov. 18, 2012, in Austin, Texas. Hamilton won the race, Vettel finished second and Alonso finished third.