While Rod Nelson believes Mercedes have romped off into the distance, he reckons Williams could have a great battle with Ferrari for second best.
Although Williams have yet to set the stage alight this pre-season both Valtteri Bottas and Felipe Massa have hovered near the front of the timesheets.
Thursday, the most recent day of testing in Spain, finished with Massa second quickest on the soft rubber, 0.408s slower than Kimi Raikkonen who set his lap time on the ultra-soft tyres.
Speaking to Autosport, chief test engineer Nelson said: "It's very close and every day's a little different.
"There's no doubt that Mercedes are extremely strong. Ferrari less so. We'd like to think we're slugging it out with Ferrari.
"We've got some more bits to bring to the car and I'm sure they have as well. Operationally we have to be as efficient as we can be.
"We'll do the best job we can. Merc's gone but we'd like to be second."
Nelson explained that he is happy Williams performance over the last few days of testing, adding that their decision to run qualifying simulations in the afternoon has had an impact on their times.
"The last couple of days in particular have been very good for us," he said.
"We spent most of last week looking at reliability stuff and doing our homework, and now we've started stretching the car a little bit more.
"We are still doing some longer runs looking at race pace and with a push towards qualifying – we're doing a one-hour quali session every day.
"We're not after headline times – we're doing the [qualifying simulation] session in the afternoon when it's generally slower because we want the best test conditions in the morning."