Both Lewis Hamilton and Valtteri Bottas struggled with the W12 while the Red Bull of Verstappen went over a second quicker than either of the pair in the desert.
Mercedes still remain joint favourites going into the 23-race 2021 calendar having won all of the last seven available Drivers’ and Constructors’ titles.
But Red Bull are hoping to dislodge Wolff and co. at the front of the grid with Verstappen the favourite to prevent Hamilton clinching a record-breaking eighth world title later this year.
And the Austrian chief admits Mercedes aren’t heading into the first race in Bahrain in the kind of shape they have entered previous season openers and have yet to truly nail down what the reason for the issues of their tricky pre-season were.
“I don’t think it’s yet there as a package in terms of performance, but I’ve no doubt we will work hard to find it somehow.
“I’m still not fully comfortable. We’ve been going forwards, I think, with the set-up of the car and understanding of the car, and getting it to behave better, but still there is more work to do.
“From these busy few days, there’s lots of data that hopefully can help us to make the car faster when we race here.”
While Hamilton reflected: “They [Red Bull] can be favourites if they want. I mean, they’re looking strong.
“They’re going to be a different machine or animal this year with, I think, a really good, strong line-up for two drivers and a really good car.”
Yet Verstappen said in response: “I know people are excited and think we are just saying this, but Mercedes are still the favourites – how can they not be when they have won seven world championships in a row.
“I’m sure Mercedes also want people to think we are the favourites and put the pressure on us but we are just focused on ourselves.
“I see every season as a new opportunity to beat them, but we would be stupid to expect that fight to be easy and look at testing and think we are ahead.”