Mercedes technical director James Allison has singled out Red Bull as potentially being the main threat to the reigning champion Formula 1 team in 2018.
Daniel Ricciardo set the fastest ever lap of the Barcelona circuit on Wednesday with the RB14, running on hypersoft tyres.
He was almost 0.4s ahead of Lewis Hamilton, who used ultrasofts on his quickest lap.
Red Bull also showed impressive reliability, as Ricciardo logged 165 laps.
"I think there's three quick teams and there's no doubt that Red Bull are going to be people that we're going to be fighting with this year," Allison told Sky Sports. "There's clearly no doubt about that.
"It's still not possible to say for sure if we've got our nose in front of them or they've got their nose in front of us, but you can say it's close."
Allison believes that there's more to come from Red Bull, saying: "I don't know what their engine plans are, but looking at what they've brought here, I'd say they've still got some bodywork to bolt on before Melbourne.
"But no one really sandbags. There's no point in sandbagging. You're here to test. And you're only going to do a valid test if you find out what your car can do.
"The thing you can do to change the absolute pace is to have more or less fuel in the car, to have the engine turned up more or less depending on what particular test you're making with it."
Ricciardo feels Red Bull is closer
"Now it looks like we're close," said the Australian. "I still don't really know where Ferrari is, I'm quite confident that Mercedes is just in front of us, but Ferrari I'm not sure. I think we're going to be a lot closer than last year.
"I don't think we're talking six or eight tenths, like it was this time last year. We seem to be close, and Ferrari I would guess are probably close, within our pace."
Ricciardo, who didn't run yesterday because he was unwell, admitted he felt tired after his marathon stint, which was planned from the start of running.
"I think if I said I'm very energetic right now, I'm certainly not, I'm running on empty, but we got through it, it was a good day," he admitted.
"Especially when the lap times are that fast, I would say a bit of adrenaline takes over, and it gets you through. But I think I'll sleep well tonight."
Additional reporting by Erwin Jaeggi