Having watched Nico Rosberg blitz the competition on Friday, Red Bull boss Christian Horner concedes Mercedes have "quite a gap" over the field.
For the first time during pre-season test, Mercedes went for a performance lap on Friday afternoon at the Barcelona circuit.
With a set of soft Pirellis on his W06, Rosberg clocked a 1:22.792 to beat Valtteri Bottas, who was also using the soft tyres, by 1.203s.
And Horner says it appears as if once again Mercedes have a sizeable advantage.
"They've been running relatively conservatively throughout the testing, but have obviously just started to wind it up a bit and you can see the performance gap is pretty ominous," he told Sky Sports News HQ.
"Everyone looks pretty tight behind them, but they seem to have quite a gap at the moment."