Christian Horner believes Mercedes' W13 appears to be "a handful to drive" although feels the team has yet to show its hand across pre-season testing in Bahrain.
Mercedes led the way at the first pre-season test in Barcelona where Lewis Hamilton and George Russell set a one-two overall ahead of the Red Bull of Sergio Perez.
The reigning champion team has yet to feature at the top of the times in Bahrain, with Horner suggesting the car may not be yet at its peak.
"Every time we see the Ferrari on track it looks competitive," said Horner, speaking to Sky Sports F1.
"I think McLaren, when it’s running, looks competitive and I think Mercedes haven’t shown their hand yet.
"The car looks a little bit of a handful to drive but I’m surprised by the Williams pace, their long-run looks quite competitive. The form book is impossible to predict."
Alluding to the 2021 season, Horner gave a reminder of Mercedes' struggles in pre-season testing before going on to win the opening race of the season.
"We were pretty happy this time last year, then Mercedes came back at the first race, won it and were very competitive," added Horner.
"It just goes to show testing is testing, there are no points for these six days, that starts in a week’s time.
"With these new regulations it’s very difficult to predict, there’s not been a lot of long runs, not a lot of race distances done.
"Tyre degradation, we’ve seen some teams struggling with brakes, that’s all stuff where, with these new cars, it’s going to be a factor."