Late on in second practice Vettel reported on the radio that the front axle of his car felt like "jelly", prompting him to return to the pits.

The team quickly got to work to try to find the root of the problem, but despite an extensive analysis, nothing obvious came up to explain what Vettel experienced.

As a precaution against Vettel facing similar worries on Saturday, Ferrari took a decision to switch chassis – which has been approved by the FIA.

The new monocoque will be built up on Friday night and scrutineered by the FIA ahead of Saturday morning's final free practice session.

Despite the difficult day, which included a spin early on in second practice, Vettel remains confident that Ferrari has the pace to threaten Mercedes in Austin.

"It was a messy afternoon, not an easy session, but the car is quick so no need to worry," he said. "The only lap I had was the one on the ultra soft.

"Before I did a mistake taking too much risk earlier in the session. I lost the rear into Turn 19, it was my mistake. So we lost that set of tyres.

"In the end, I felt something was not right with the car so we came in a couple of times to check, then go out and double check."