Felipe Massa admits he cannot say with 100 percent certainty that Williams have the same engine as the Mercedes works team.
Williams finished the 2014 season as the fastest car behind Merc and many backed them to keep that position at the start of the season opener in Australia this weekend.
However, they were surpassed by Ferrari at Melbourne with Sebastian Vettel claiming the final podium spot and Massa only fourth, and Paddy Power has now made the Italian outfit second favourites in their latest market offering.
Massa admits Williams will need to have the best possible engine available if they are going to keep Ferrari behind them.
"We're pushing hard with the engine which I'm sure has some improvements that we can have and are pushing to have, because for sure the difference is too big," he said. "So I really hope that we really have the same engine, which I really don't see why we don't have.
"Why would Mercedes help Ferrari? So we need to have the best they can give, and we will work on that. On the car we know that every race and whatever points you're not getting, it counts.
"If we don't have the same engine – which I cannot say 100% because it's very difficult to say – but if we don't have it then we want to have it because it's not nice that we don't have it."
Ferrari, though, have made major strides during the off season with their power unit as it wasn't only the Scuderia who impressed in Melbourne as customer outfit Sauber finished fifth through Felipe Nasr.
Massa admits Williams now have to keep an eye on the likes of Ferrari and you can bet on this more at Paddy Power.
"Who finished fifth? Sauber. From where they were last year and not having any money to put in the car. It's a team that doesn't invest so much in the team and they just improved a lot which I think is coming from a different part," he said.
"If you see how we finished the season and how they are now, it's a big step, for sure.
"Apart from the lap time you just look at the speed. At the end of the year we were 10, 12 or 15kph quicker than Ferrari, now they are similar and maybe even a little bit better than us."