Fernando Alonso admitted he "couldn't believe it" as another Honda engine problem forced him to retire from the United States Grand Prix this past Sunday.

The Spaniard had been pushing his McLaren to the very limit all weekend, as he looked to make up for a near 20kph top speed deficit down the straight through the corners, his efforts were rewarded with a top 10 grid slot in qualifying but it would all come to nothing in the race.

Despite the obvious disappointment, Alonso, who has only seen the chequered flag at six of the 16 races he has entered this season, was at least satisfied with his own performance.

"I'm very happy with how the weekend went on a personal level. I think the level this weekend was sublime, both in qualifying and in the race," the 36-year-old commented.

The main problem this season has been taking the often lofty positions the double world champion has been in during races, at least compared to his teammate, and converting them into results with just two finishes in the points in Baku and Hungary.

"The engine was losing power and failing and I couldn't believe it,” Alonso said of his retirement in Austin. “In Japan, you start last, you are running 15th or 16th after the penalty and nothing happens, the engine works perfectly all race. Now you are running seventh and it starts to fail and you can't believe it.

"The championship standings don't reflect that (the chances that have been squandered)," he added, "it doesn't change much for me to be 12th or 17th in the championship but on merit, I think we should be a lot higher up."

Declaring the engine from Sunday as "broken", Fernando now expects another fresh unit will be needed this weekend in Mexico and with it, will come another grid penalty, but he is not giving up on the final three races even if there's very little to be gained.

“Obviously all attention is placed on next year but in these three races I'd like to score points to improve in the standings and to help the team as well,” Alonso said.

"But from the three races remaining in one we'll start last, so let's see if we can score in the other two."