Felipe Massa believes that three team are now comfortably ahead of Williams after the first three races of the 2016 season.
The Brazilian driver who, along with team mate Valttero Bottas, has scored consistently since the season-opening Australian GP, was hoping his Williams FW38 would demonstrate more speed compared to its rivals.
Hopes were particularly high for last weekend's Chinese GP as the venue appeared well suited to the FW38's qualities.
While Massa noted the team's improved strategy, which helped him take sixth place, he believes the gap with Mercedes and Ferrari has increased and that Williams now have to contend with a revived Red Bull.
"Three teams are in good shape, better than us," Massa told Brazil's Globo.
"We can get a little closer - depending on the track - to Red Bull in qualifying. But we know that in the race they have a much stronger car.
"I finished in the best position I could. We really need to understand where the other cars are better than ours.
"We know it will not be easy, but we will continue to fight and to work."
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