Pat Symonds has admitted Williams' next step will be more difficult than the giant leap of 2014.
After a career slump that hit bottom in 2013 with ninth in the constructors' standings, the once-great Grove team managed to end last year as arguably Mercedes' closest challenger.
But Red Bull designer Adrian Newey thinks Williams was helped last year by its switch from Renault to industry-leading Mercedes power.
"Clearly, last year was engine-dominated," he told Sky, "and we saw that with Williams springing forward.
"Obviously Williams did a very good job but I think what really made the difference for them was the engine."
Williams' technical chief Symonds, however, said the British team is proud to have beaten a grandee name like Ferrari to third place in the constructors' standings.
Stable, but not rich
"We are stable and do not have financial difficulties," he insisted, according to France's L'Equipe, "but we do not have the budget of some of our competitors.
"So it was especially pleasing last year to beat Ferrari for example, as we have half the budget they do. But to be better than third in 2015 will certainly be more difficult than going from ninth to third place last year."
Symonds also denies that Williams' leap forward can so easily be explained by its superior 'power unit'.
"In 2013," he pointed out, "we also had the same engine as the world champions (Red Bull) and we were ninth. We had many new things in 2014 including the aero and the braking system, and those things do not happen by magic.
"We want to continue on this path in 2015," Symonds added.