Williams have work to do ahead of the Chinese Grand Prix to ensure Ferrari do not disappear into the distance, says Valtteri Bottas.
Sebastian Vettel's victory in Malaysia confirmed the Italian team as major challengers to Mercedes for the constructors' title this season, while Williams are yet to show the form from testing that suggested they could also be contenders.
Bottas and Felipe Massa regularly impressed with their pace ahead of the new season, but have collected only 30 points from the opening two races.
"In an ideal race, starting at the front, it [the gap between the teams] wouldn't be that big, but it is still much bigger than we expected," Bottas told Crash.net.
"It shows that we have a lot of work to do and there is much more potential, we need to get more power from us. We will have some updates coming and all we can now learn from anything we can, to just keep pushing to the maximum.
"Ferrari seem better than anyone with the tyre degradation, so we will see in China, but I think they are in front of us. We need to keep working hard if we want to challenge them."
The Formula One season resumes on April 12 in Shanghai.