Mere days into 2019 winter testing, it appears that Ferrari has a clear advantage.

With Sebastian Vettel and Charles Leclerc smilingly topping the times, the most obvious difference between their red car and the others is the front wing.

Toto Wolff, the Mercedes boss, admits that visibly, the wing produces less downforce.

"Somehow they must be compensating for that. The question is how," he told Germany's Auto Bild.

An even more extreme version of the Ferrari wing can be seen on the Alfa Romeo, a team closely aligned with the Maranello marque.

"The concept looks wrong, but we'll have to try it out in the wind tunnel," one engineer said.

But Wolff said it would be the wrong approach to "blindly" copy another team.

Red Bull boss Christian Horner agrees: "It wouldn't work to bolt a Ferrari wing onto our car."

Franz Tost, the boss at Toro Rosso, said an analysis of the current difference between Ferrari and Mercedes indicates a 0.5 second per lap advantage for the red team.

"At least," he said. "I think it's more than that."

Another theory is that Ferrari's secret is deeper than just the wing. Auto Bild said the rear of the Ferrari appears to lower at high speed.

FIA technical chief Jo Bauer is quoted as saying the Ferrari is "absolutely legal".

Daniel Ricciardo, who drives for Renault, observed that Ferrari is so far "the only team that has sent a strong message" after the two days of testing.