Fernando Alonso has piled more pressure on struggling McLaren and partners Honda by saying it is 'realistic' to think they can catch up with Formula One champions Mercedes next season.
The Spaniard told reporters at the US Grand Prix, where he was ninth fastest in a wet first practice on Friday, that McLaren needed to find another two and a half seconds of pace to match Mercedes.
"If we improve two and a half seconds, we will win next year," he said.
"It's realistic, but the thing is we need to work very hard with very high discipline all winter, all united, because in this new Formula One everything is a package, it's not anymore an engine, aerodynamic, mechanical side," he added.
"It's all combined that makes the perfect car. We are working on that, we understood that, and we are trying our best. It's a very big gap that we have to close."
McLaren, the second most successful team in the sport's history, are enduring their worst season this year and are ninth of 10 teams.
Alonso, the double world champion who joined from Ferrari at the end of last season, has suffered frequent car breakdowns and grid penalties due to the team far exceeding their engine allocation.
The Spaniard again has Honda's latest engine upgrade, which he first used in Russia two weeks ago and said he expects to see a step up in Austin on Sunday.
"The data shows that the improvement on the engine is as expected, so this is very good news," he said, while mindful of the weather conditions.
"I'm happy with the direction we are taking. More important than this step is the philosophy for next year's engine that this step or spec has introduced. This is a good direction to go," said Alonso.
Fellow McLaren driver Jenson Button, who has the older specification engine, said he hopes to be 'annihilated' by his team mate because that would show Honda had made improvements.
Alonso said Honda were optimistic about next year's engine.
"I think they learned many, many things that they didn't expect...I think the steps that we can do are much bigger than any other team because they are more or less in a situation that they knew already and they know perfectly," he added.
"For us, every lap is new information that we are learning and that we are taking into account for next year."