Red Bull/Ferrari battle confusing - Ricciardo

Ferrari trails Red Bull by 50 points with four races remaining in the battle for second in the constructors' championship, but the Italian team showed improved pace on the sweeping turns of Suzuka last time out.

Sebastian Vettel and Kimi Raikkonen recovered from grid penalties to finish fourth and fifth respectively while Ricciardo was sixth and his team-mate Max Verstappen second.

Ricciardo conceded he was surprised by the pace demonstrated by Ferrari in Japan and suggested the pace of both teams at certain races had not transpired as expected.

"Ferrari have been extremely fast lately," he told reporters at Austin.

"In Japan, they did a very good job, they were pretty quick, probably quicker than we expected.

"It's been a bit like that - some places we have been quicker where we didn't expect but then we have gone to places like Japan where we thought we would be quicker than them and we weren't.

"So it is still a bit confusing for everyone to know who has the better package and what tracks suit who, but maybe they just got it right that weekend and put it altogether.

"I haven't watched the complete race back but it looked like [Vettel] had a car for the podium, so they probably underachieved what the car was capable of."

Ricciardo believes Ferrari will pose a threat in America, particularly as the first sector is a combination of high-speed corners like at Suzuka.

"I expect them to be pretty quick here," he said. "It's going to be close.

"We'll have our work cut out if both our cars want to finish in front of them."

Verstappen added: "They will be strong, the last few races they have been strong and perhaps sometimes a bit unlucky.

"I'm not too worried, we still have a great car as well and as you could see in the race [in Japan], we had very good pace."

Meanwhile, Ricciardo said he was hopeful Red Bull had got to the bottom of the straight-line speed problems he encountered in Japan.

"We found out a bit in the end, there was a bit with the fuel flow meter, they are still a bit inconsistent," said Ricciardo.

"You see even across cars and that, some have a little bit more on them than others and it is just sometimes luck of the draw.

"It looked like I was grounding a bit more, scrubbing off a bit more speed on the straights, so a couple of things which probably emphasised it.

"Both of us have new power units here, our fifth and last one for the year. Hopefully it's all good again."