Red Bull motorsports advisor Dr Helmut Marko has conceded to being wary of Mercedes' 'dangerous potential'.
Mercedes has been unable to match Red Bull and Ferrari for pace across the opening seven races of the season.
But in practice at recent races in Miami and Spain, Lewis Hamilton and George Russell were able to close the gap to the frontrunners.
“Mercedes is doing incredibly fast laps in practice and also in the races, both with Hamilton and Russell,” Marko told Formel1.de.
"Not continuously, so for us it’s difficult to interpret where that lap time is coming from. Listening closely to the Mercedes people, I don’t think they know either.
“But the dangerous thing is the potential seems to be there. And once that can be called up over a race distance, then they’ll be right up there or right at the front.”
Mercedes struggles 'not a bluff'
Both Russell and Hamilton have been able to extract strong pace from the W13 at times in race-trim but neither has managed to maintain the speed.
Asked if Mercedes has been 'bluffing about its speed all season, Marko added: “No, it’s not a bluff.
"The bluff was where they had this weird engine advantage and last year the Hamilton engine – there they were bluffing hard. So, I don’t believe that.
“And it’s really incredibly hard to understand where this [car is] incredibly fast. They were the fastest in the field at [one] stage, but it doesn’t last long.
“What exactly changes there, why they can’t drive that continuously, is just – thank God – still the problem, I would say.”