Dr Helmut Marko thinks Red Bull-Honda is Mercedes' "first challenger" in 2020.

The Red Bull official was speaking before the final three days of winter testing in Barcelona this week.

"We will only get a clearer picture this week," he told Auto Bild.

"We'll be driving the Melbourne version on Friday and I assume the others will do the same. But I am convinced that we are Mercedes' first challenger.

"Ferrari has always driven with a lot of fuel and a low engine setting, but the statements from Sebastian Vettel and Charles Leclerc show that they were not where they had hoped in the first week of testing," Marko added.

He thinks the gap to Mercedes is just two tenths of a second.

"They make up that time because they have no understeer in the tight corners. They do that with DAS," said Marko, referring to Mercedes' trick steering system.

Mercedes stunned the paddock with the DAS innovation last week.

"Incredible," said Daniel Ricciardo. "You'd think it would be the teams that are behind them, not the team that has been dominating, who would be doing these things."

But Mercedes' technical boss James Allison said the team needed to be bold.

"It was a strong temptation to continue to develop the old car, but we understood that a conservative approach would not be enough," he is quoted by La Gazzetta dello Sport.

"So we decided to build an aggressive car with impressive solutions."