Williams believes it will be "scrapping" with Ferrari early in the 2015 Formula 1 season and suspects the Italian team is closer than final testing results suggest.
Despite finishing behind Red Bull in third in the constructors' standings, Williams was world champion Mercedes' closest rival on pace in 2014.
It was Mercedes' nearest challenger again in the final test at Barcelona last week, ahead of Ferrari.
However, Williams's chief test and support engineer Rod Nelson believes a mistake by Sebastian Vettel hid Ferrari's true potential.
The German's super-soft tyre run on the final day of testing was four tenths of a second slower than Williams driver Valtteri Bottas's effort on the same rubber, but Nelson thinks that was deceptive.
"I think he pushed too hard at the start of the lap and lunched the tyres by the time he got halfway round," said Nelson, when asked by AUTOSPORT where Williams is in the pecking order.
"So I expect Ferrari to be a little it closer than maybe it looked.
"We're going to have a scrap with them, certainly at the start of the season."
He admitted that Mercedes looks out of reach, having beaten Williams and Ferrari's super-soft times on soft tyres.
"Mercedes are looking incredibly strong. I'm sure its pace made a lot of people sit back a little bit.
"It certainly made us think twice, but we should be in the hunt for podiums."