Helmut Marko says Red Bull experienced in Bahrain last weekend its best pre-season test ever in F1, but the Austrian still labels Mercedes as the championship's outright favourite.
Red Bull's Max Verstappen set the pace at Sakhir while teammate Sergio Perez was also among the front-runners during the three-day test.
The Milton Keynes-based outfit's speed and consistency displayed in Bahrain bode well for its prospects in next week's opening round of the 2021 season, especially considering Mercedes' woes at Sakhir marked by the reliability and handling concerns over its W12 car.
In an interview with Germany's RTL, a buoyant Marko hailed Red Bull's early form but the Austrian tipped Mercedes to bounce back strong, suspecting the reigning world champions to have "something up their sleeves".
"That was certainly the best test since Red Bull Racing came into existence," said Marko. "Everything worked right from the start.
"But we know that Mercedes is the favourite, and I assume that they drove with much more fuel than we did.
"The fact that they didn't really align with others in terms of fuel load, that already shows that they must have a lot of confidence. So we assume they still have a lot up their sleeves.
"Mercedes is the favourite and we are the first challenger. That's how I see the situation.
"Behind them, it's going to be a battle. I think McLaren will stand out a bit, but not at the level of Mercedes and us. The midfield is certainly closer together."
Mercedes' 2021 car was described as "snappy and unforgiving" by Valtteri Bottas, a description that certainly fit the bill based on last weekend's trackside observations.
In contrast, Red Bull's RB16B appeared incomparably more stable and as an all-round more predictable car, as the team projected.
"Compared to Mercedes, our car definitely seems calmer," Marko said. "Of course, that makes it easier for a driver. It's predictable, and you can work better at the limit. But that was also the goal.
"We didn't want to make the car slower so that it's easier to control. We wanted a fast car that is predictable. And I think we achieved that well."