Ferrari have paid tribute to Fernando Alonso's "great achievement" in finishing second in the drivers' championship in their car.
Team boss Stefano Domenicali said: "Being second with the problems we had, above all in the second part of the year, means he did a great job."
Ferrari have been slower than Red Bull, Mercedes and Lotus for most of the year and are now struggling to beat Sauber.
Alonso said: "With the car we have, it is like world champion of the rest."
The Spaniard was a threat to Sebastian Vettel in the first half of the season but has been left behind in the second half as the German romped to eight consecutive wins and the world title.
Alonso has now finished second to Vettel in three of the last four seasons.
Alonso sealed his position with fifth place in the US Grand Prix, a finish he secured after dropping to seventh on the first lap before fighting hard to pass McLaren's Sergio Perez and Sauber's Nico Hulkenberg.
"It's getting more and more frustrating because for whatever reason we are losing a bit of pace," he said. "Now entering in Q3 [the top 10 qualifying shoot-out] is difficult, entering in the points is difficult."
Alonso said he was disappointed not to have beaten Mercedes driver Lewis Hamilton, who finished fourth, because it meant Ferrari lost further ground to Mercedes in their bid to finish second in the constructors' championship.
Ferrari are also under threat from fourth-placed Lotus, who are 18 points behind after Romain Grosjean finished second in Austin on Sunday.
Alonso said: "The main goal was to beat Mercedes and it was not a good weekend from that perspective and Lotus are getting closer.
"We are losing pace and we need to find something for Brazil because it is going to be a tough weekend.
"We need to do better if we want to fight for second place in the constructors' championship or maybe that was a too-optimistic goal.
"We need to be more realistic and know fighting for second in the constructors was a dream and maybe that dream is difficult to come true."