Lewis Hamilton, Mercedes F1 W15, walks away from his car after retiring in the gravel

Mercedes thinks that its rollercoaster United States Grand Prix form was triggered by a set-up that 'flirted' with the ground too much.

The German manufacturer was left scratching its head at the end of the Austin weekend as it struggled to understand why it had been so competitive in sprint qualifying, with both cars in contention for pole, and then struggled so much the rest of the weekend.

Adding to its woes were identical spins that George Russell and Lewis Hamilton suffered at Turn 19, one in qualifying and one in the race, which fuelled questions over whether an upgrade fitted to the W15 had trigged some unexpected problems.

It has left team boss Toto Wolff convinced that its pre-summer form - when it won three races out of four - is now long gone and the squad is once again adrift of McLaren, Ferrari and Red Bull.

"We're back to underdog status," said Wolff. "We are not back to the pre-summer situation. It's not about coming into the weekend and thinking we're going to win this.

"It's more going into the weekend and thinking at the moment we're the fourth team on the road."

While the team is still working on understanding its car better, James Allison thinks that there is a simple explanation for what happened in Austin - and it revolves more around its set-up choices than anything else.

Speaking in Mercedes' regular post-race debrief about what had gone wrong between Friday and the rest of the weekend, he said: "That is the million-dollar question.

"Not only did George nearly secure the pole in that [sprint pole] session, but Lewis also nearly smashed it utterly out the park. He was way ahead of any of the times until he got a bit tangled up with [Franco] Colapinto.

"Why that did not materialise in the rest of the weekend, is the key question for us. My guess is that we were flirting a bit too closely with the ground.

"These cars like running low and you generally pick up lap time as you can get the car nearer to the ground. But, push it too far and the car starts behaving in an unpleasant fashion.

"If you just hit a bump wrong, it will unseat the car, make the rear end come out on you and just deliver a level of performance that when it is good, it is great.

"But if you just hit a bump at the wrong moment or a crosswind at the wrong time, then you get punished for it. My guess is that we were just pushing our luck a little bit too much in terms of how near to the ground we got, how stiff we ran it."

While Hamilton suggested after his early race exit that the team needed to question whether or not to continue with the latest upgrade package, Allison says there are no doubts it is a step forward.

And, while acknowledging that the spins suffered by Hamilton and Russell were unusual, he does not think they were triggered by new parts.

"All the things we measured suggested that the upgrade package was behaving as we would expect," he said. "The downforce was there.

"We did not see prior to the spins anything breaking down on the flow structures under the car. There is every reason to think the updates were good and we will enjoy that for the remainder of the year.

"The spikiness that we saw that pitched us into the barriers on an unusual number of occasions was because we had the car too low, too stiff, and had the handling degrade unacceptably as a result."