George Russell admits to being impressed by both Audi and Haas as the pre-season shakedown got underway.

The Briton and his Italian teammate were both in action for Mercedes over the course of the opening day, unofficially posting the second and fourth best times, respectively.

"It's amazing just to be back out driving again," Russell told F1TV. "I think at the end of the year everyone's looking forward to a break, but then after a week or two you're raring to go again," he admitted.

"There was obviously a lot of anticipation for these new power units, and so far, just generally on the whole, the amount of power they give is pretty impressive," he said. "I think for all the cars I was watching today, with all the different power units, it's probably the quickest I've ever seen an F1 car pass here in Barcelona, so that was pretty exciting to see.

"And the cars, you feel it being smaller," he continued, "you can feel that weight reduction compared to previous years, so I think from that aspect as well it's gone in a good direction.

"For sure, we're very pleased with the day," he added, "but I was pretty impressed with a number of other teams, to be honest. You see the Red Bull-powered teams' brand new power unit and they're like a brand-new team from a power unit side, and they had a really smooth day with two cars.

"Audi had some good laps in there as well, and I think Haas did the most laps of everyone with a Ferrari engine. It's not quite 2014 vibes of half the grid is breaking down and having loads of issues, I think Formula 1's evolved so much since then and the level's just so high.

"From every single aspect, it was pretty impressive to see all the teams on the whole having a lot of laps under their belt on day one."

At this stage times are pretty unimportant, it is reliability and therefore the number of trouble-free laps that really count.

With that in mind, despite Audi impressing Russell, a technical issue meant that Gabriel Bortoletto only completed 27 laps, while between them Cadillac's Sergio Perez and Valtteri Bottas completed 44.

With 154 laps under his belt, Esteban Ocon will have left Haas feeling very happy, likewise Red Bull, where Isack Hadjar completed 107 laps on his way to posting the best time of the day.

"We were doing a pretty decent job in the morning," Bortoleto told F1TV, "putting some laps onboard, testing some things, we were going in the right direction. But, as we all expect, this is a shakedown and everything can happen, and we expect to find issues here and there in the car. And we found a couple of problems that unfortunately put us out of the day, basically, from the morning already. So I didn't run really much today.

"It was expected, this type of things, you know, we're finding everything now to not hopefully find it in the next test or even in the first race of the year. Overall, I would say when we were running, positive, good to have a few laps with the new car, the new regulations, power unit and everything. So hopefully on the next day I'm driving, I can get a few more laps on board."

"We had a technical issue with the car," admitted Audi's Jonathan Wheatley. "We spotted it, decided to switch the car off on track."

"I really enjoyed getting back on track," said Valtteri Bottas, "this was the first time I drove this car, so it's just good to be out there."

Asked about the relative lack of laps, he said: "It's debugging, that's why we're here and it seems like every team had some issues.

"It's just now really figuring out those and then getting more and more running each day, but it's just great to be here and in the car."

Though happy to be back in the office, the Finn admits that a season away and the rules overhaul leaves him with a steep learning curve.

"It's definitely a big difference to before," he admitted. "The cars, they handle different, you have a bit less load, especially in the high-speed corners. The PU, you have so much more torque out of the corners, but then you have the battery to manage and again, for Ferrari, this is the first proper day of running with their power units, so naturally there is a learning curve."

"New cars, new power units and very, very different to drive," added Liam Lawson, "so we're just adjusting at the moment, but so far all good.

"I definitely haven't got my head around it fully yet," he admitted. "It's something that we'll keep learning over the next few days and weeks when we go to Bahrain as well. But it's very, very different.

"It feels like there's a lot more we can do as drivers potentially to make a difference, which is good," he added. "But right now, it's very early days. So, it's very hard to know where we are. But for now, just trying to learn how to optimise the car. I'm enjoying it so far.

"The main thing is reliability at the moment," he explained. "We've done a good number of laps today, only a few little things, and honestly, the only issues we really had today were safety precautions, not really any actual issues.

"So far, honestly, on the power unit side, it's been very, very good. But again, it's hard to know compared to everybody else where we're at."