Williams believes Scuderia Toro Rosso currently has the upper hand in terms of speed as last weekend's Australian GP demonstrated.
Williams' ambitions from the outset this season are to be the 'best of the rest' behind Mercedes and Ferrari while attempting to close in on the two front-runners.
But in view of the team's relative performance in Melbourne, Toro Rosso will not only be a rival the Grove outfit will have to contend with, the team's STR11 also appears to be able to outrace Williams' FW38.
Williams technical director Pat Symonds believes qualifying offered a clear view of Toro Rosso's current superiority.
"In testing, we were not completely clear where Toro Rosso were," Symonds told Autosport.
"When we rank the cars, we put error bars on our rankings because it's not perfect data and we had put error bars which were more negative than positive on the Toro Rosso and that was a mistake.
"We should have made them symmetrical and then they would have been pretty good. Toro Rosso is the one which has really surprised us. The others are quite close to where we thought they would be."
In Symonds view, Williams has fallen behind Toro Rosso in the current hierarchy, although it's still early days.
"We probably see Toro Rosso, then ourselves, then Force India. I wouldn't like to separate ourselves and Force India and Red Bull.
"Toro Rosso are faster than Red Bull. Red Bull played their cards well in the race [to finish fourth with Daniel Ricciardo], they made the most of it.
"But it's a sample of one race, we'll see how things change on different circuits."
Scene at the Australian Grand Prix
Australian Grand Prix - Quotes of the weekend
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