Lewis Hamilton and Mercedes Formula 1 team boss Toto Wolff both believe Ferrari has lost straightline performance at Austin, but stopped short of attributing that to the FIA's fuel-flow directive.
The governing body has issued a technical directive in response to an enquiry from Red Bull Racing that explored possible loopholes in the measurement of fuel flow.
It was intended to test a theory about what Ferrari might be doing, a ploy that teams regularly use when they have questions about rivals, amid the ongoing questions over the size of Ferrari's straightline performance advantage.
The FIA duly confirmed that the systems proposed by Red Bull would not be legal.
On Saturday Valtteri Bottas gave Mercedes its first pole position since July, and Ferrari appeared to have a less significant straightline gain compared to its main rivals - though its drivers did still top all the speed trap tables.
"What's come out this week is quite interesting to see," said Hamilton.
"Obviously today, I think they've lost a bit of power. It will be interesting to see how that continues or how that reflects in the race.
"They are still quicker than us generally in the straights. Hopefully our car will be strong throughout the race."
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Wolff declined to suggest that the technical directive had led to the change in comparative performance in qualifying between Ferrari and main rivals Mercedes and Red Bull-Honda.
"Certainly the three teams were much closer together in terms of straightline performance here in the US," he said.
"But I wouldn't say this is down to any specific event.
"It's just a fact that we've won our first pole position since July, since Hockenheim, and we were really in the mix."
Asked by Autosport for his thoughts on the FIA ruling, Wolff replied: "I think it's very strong that the FIA issued a TD clarifying the situation, with some very clear wording.
"But this is a process that is standard, it's happened before, and part of the role of the governing body."
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