Sebastian Vettel has said that he expected Williams to be putting up a tougher fight in 2016.
The Grove-based team finished third overall in the past two world championships - behind Ferrari last year, and Red Bull in 2014.
Both of those teams are ahead of Williams so far in 2016, and Ferrari's Vettel told UOL Esporte: "Maybe the surprise is that Williams is a bit behind so far.
"Perhaps this was the biggest surprise."
Brazilian Felipe Massa, however, is less surprised.
"We cannot forget that we are fighting against teams with more potential than us," he said, referring to the much bigger budgets wielded by Mercedes, Ferrari and Red Bull.
"We are trying to improve the car but I think [Williams's rivals] always had a better car than us. The difference is their engines are improved now.
"So we know it's not so easy to develop against teams as strong as Mercedes, Ferrari and Red Bull. Even with McLaren it's not so easy, but we are in the fight."
The championship continues with the Monaco Grand Prix next weekend.
