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Cape Town - Williams chief Pat Symonds admits the British team is no longer Formula 1's third force.
In 2014 and again last year, Williams finished the constructors' championship in third place, behind Red Bull in 2014 and Ferrari in 2015, as engine supplier Mercedes dominated in the new 'power unit' era.
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Budget constraints
Technical boss Symonds said: "Either one of them (Red Bull or Ferrari) was in front of us or the other, but at the moment it's both.
"When you've already come up with lots of clever things, it is incredibly difficult with our budget to keep up with teams that spend twice as much," he told Germany's Auto Motor und Sport.
According to Felipe Massa, the main difference in 2016 is the step forward taken by Red Bull.
Massa said: "In the corners they've always been good but now they also have a good top speed."
And he told Brazil's Band News FM that he is expecting Red Bull to be even stronger in Barcelona this weekend.
Massa continued: "We know it's a track where they will be better than in Russia, so we need to keep improving to be in the fight with them.
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High hopes in aero packages
It was hoped a recent update, including an ultra-short front nose, would give Williams a boost, but Massa said the team is still working on perfecting it.
"The aerodynamic engineers claim the parts are better, but we need to learn how to put down on the track what the wind tunnel promises," he said.
The good news for Williams is that the Grove-based team has proved the fastest of all so far this year when it comes to pitstops.
While a good stop in 2015 might have taken the crew 3.3 seconds, Massa said: "Now we are talking about a bad stop if it takes 3.1 seconds.
"In Melbourne I even lost a bit of time because I was surprised that the guys had finished so soon, now I'm used to it," he concluded.