Russell had a full day in the car in Bahrain on Wednesday, before Hamilton took over on Thursday.
While the team's programme meant that Russell's headline times were modest relative to those of rivals, he says that it was already clear that the car is an improvement.
Russell and Hamilton both lacked confidence in the rear end of last year's W14, and crucially that trait appears to have been addressed.
"The car last year was really challenging to drive," said Russell.
"Lewis and I had no confidence in it, it felt like it was going to bite us every single corner.
"We can attack the medium and high-speed corners without the rear end snapping out. And we feel like we've made a really good step in terms of the consistency of the car.
"We can really lean on it better than we've been able to in the past. And this was a huge focus throughout last year.
"We saw many flaws with the W14, which the team have done a really great job to rectify.
"And we've now got a car mechanically where the aero guys can go in focus on just building downforce upon that, whereas in the past, whatever we did aerodynamically there were underlying issues with the race car that took a while to understand, and so on."
Asked what the team is still missing, he said: "More downforce. When the car feels nice, but it's not quite on the pace, you just need to find downforce in the right places.
"With this iteration of regulations, there's definitely a sweet spot for all of the teams. You want the car as low as possible, but you can't go too low in case you're bottoming out or you cause bouncing, which is still a little bit there in the background for some teams.
"And it doesn't take a lot to find that sweet spot and find a lot of performance. Hopefully Red Bull are already in that sweet spot, and we can close that gap. But yeah, it's going to take a lot of hard work to do so."
Although he felt good in the car Russell is under no illusions about the strength of the opposition that Mercedes faces.
"You can definitely tell from the very beginning if the car is an improvement and if it's nice to drive," he said.
"And it definitely is an improvement, there's no doubt about that. But you could have the worst car to drive, but if it's faster than everybody else, you'll be happy with it.
"So it was definitely pleasurable to drive yesterday, I had a good feeling within the car. But we know that all of the other teams have made a good step forward.
"Right now it's definitely far too early to say and we have a huge, huge mountain to climb to catch up with what Red Bull was doing last year, and how far ahead of everybody else they were.
"We need to wait and see. But we've definitely got a much better platform to build upon, and it's not the diva that it was in the last two years."
Asked about an impressive first day that saw Max Verstappen's Red Bull RB20 clear of the field by 1.1 seconds, he said: "Probably as you'd expect from Red Bull, they're in such a great momentum at the moment.
"They started these new regulations on the front foot, and they came here and have been solid ever since. They definitely are favourites, they are definitely a step ahead of everybody else here in Bahrain.
"They've had an impressive winter, no doubt, but that was to be expected. Such an impressive team and outfit. And we need to see where we fall out in the coming races."
