Red Bull's Christian Horner has pointed to Lewis Hamilton's immense straight-line speed in Brazil as being particularly worrying, after the reigning World Champion took just 24 laps to cut his way from last to fifth place in the Sprint Qualifying race.
Hamilton had no issues slicing his way through the pack at Interlagos, including easy passes on Mercedes-powered rivals, and Horner has revealed that there are doubts within the Red Bull camp as to what's causing it.
"Lewis' straight-line speed is just mind-boggling," Horner admitted to Sky Sports.
"We've seen this coming for a few races now. Turkey, it started to be there. In Mexico, we saw the straight-line speed, I think they were about 14 km/h quicker at the end of the straight than we were, so it's no great surprise.
"We saw yesterday that it's phenomenal, especially with the size of the rear wing that they have on the car."
With Hamilton disqualified from qualifying after a breach of the technical regulations regarding the Drag Reduction System (DRS) on his rear wing, Horner said his team are doing their best to discover Mercedes' secret recipe for their straight-line speed.
"It's something that is enabling the car to do that kind of speed," he said.
"Something must happen, because physics wouldn't allow.... The kind of horsepower delta that you would need to achieve that, would be pretty significant. So we're obviously trying to understand what it is and go from there."
Horner hints at the rear wings
With Red Bull allegedly approaching the FIA on Friday to request clarification regarding flexing of the Mercedes rear wing, Horner was asked about the early-season drama this year when Red Bull were forced to change their rear wing design due to excessive flex down the straights.
"Obviously, a technical directive came out that changed the testing procedure on that," he explained.
"The wing was revised accordingly, [but] maybe we need another change to the test. That's Formula 1, that's what the business is all about."
With the technical rules being so restrictive in areas that ostensibly prevent a huge disparity in outright top speed, Horner said Hamilton's speed during the Sprint Qualifying raised some concerns.
"It's down to the FIA to police. The rules are quite concise, they're ever more constricted," said Horner.
"It's something that we're going to keep an eye on because, particularly with Lewis today, that was just a different league. I think he was 27 kilometres an hour faster than Lando [Norris] when he passed him, which is another formula, but it's a great drive from him."
As for what it could be that's making the Mercedes so fast, Horner smiled and said: "I think we very clearly know what it is. That's for the engineers to discuss."
Video: The secrets of the Interlagos Circuit
Follow Rudy van Buren as he takes you on a flying lap around the Interlagos Circuit, home of the F1 Brazilian Grand Prix.