Williams believes Ferrari's Formula 1 power unit is a "huge step forward" after the Scuderia's performance in the Australian Grand Prix.
Sebastian Vettel finished third on his Ferrari debut while the Ferrari-powered Sauber of Felipe Nasr finished fifth to mark a big improvement for both teams on last season.
Ferrari was fourth in the constructors' championship in 2014 and only scored two podiums while Sauber failed to score a point for the first time in its history.
Williams performance chief Rob Smedley said: "If you look at the two teams who have made a huge improvement, it's Ferrari and Sauber.
"And what's the common link? It's pretty clear that the Ferrari power unit has taken a huge step forward."
When asked if the difference in race pace between Williams and Ferrari was down to the Scuderia's power unit, Felipe Massa said: "Yes, 100 per cent.
"Who finished fifth? Sauber. Where were they last year?
"It is a team who don't invest so much in the team and they have just improved a lot.
"If you see where we finished the season and where we are now, [the Ferrari power unit] is a big step. Just look at the lap time.
"And at the end of the year, we were 12-15km/h quicker than Ferrari. Now they are similar, maybe a bit quicker than us."