Williams' technical chief Pat Symonds has admitted that he was surprised by how fast Mercedes were at the final pre-season test.
The Silver Arrows proved to be quicker on the slower soft tyres than some of their rivals were on the faster super-soft rubber.
However, Symonds still believes that Williams are in a good position heading into the season-opening race in Australia next weekend.
He is quoted by Autosport as saying: "I certainly don't think we're in a bad position and I think maybe by the time we get to China or Bahrain we'll start to know where we are. Melbourne's always an odd circuit to try to establish the order from.
"I think we've worked hard, we've produced a good car, we certainly haven't dropped off from where we were last year and we've continued the solid progress we were making last year.
"I think that's very gratifying so eventually we've got to get up there and beat them."
Williams finished third in last year's constructors' championship, which was won by Mercedes.