Lewis Hamilton feels the Mercedes W07 is not in the 'right ballpark' on Pirelli's soft tyres yet, and reckons the compound is a step back on last year's Formula 1 tyre.
The three-time world champion turned his first low-fuel laps on the compound on Wednesday morning, following a race simulation on Tuesday afternoon and extensive work on F1's medium rubber.
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While Hamilton set the fastest time on Wednesday morning, he noted "some work needs to be done balance-wise", and was not happy with the revised construction of Pirelli's soft tyre.
"I'm not feeling great, those tyres are not particularly good," he said of the run.
"I've been pounding around doing multiple laps on long runs, which is not the most exciting.
"To get to light fuel is always an exciting feeling, but it's always new when you get back to pushing for a single lap.
"The car is in a different ballpark, not the right one at the moment, and that's what we need to work on.
"I genuinely wish we had last year's tyres, because these ones aren't as good.
"Otherwise it was a good experience to get onto the softer tyre."
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Williams pair Valtteri Bottas and Felipe Massa, and McLaren's Fernando Alonso have expressed views that Mercedes remains a clear step ahead entering the 2016 season.
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Hamilton said he had not taken any note of timesheets, other than what he had been told by his engineers, and urged caution on buying into the "hype" around the potential for another season dominated by Mercedes.
"Honestly, I haven't looked at where everyone else is," he said.
"Everyone is naturally going to hype us up, every single person is going to hype us up.
"But you really don't know what fuel load Ferrari is on, I think they have a strong package. Even Williams looks like it has quite a strong package.
"I wouldn't take any notice of what they were saying, to be honest."