The British team started testing in Spain late because of build delays with its new FW42.

Russell kicked off the second test on Tuesday by completing 119 laps, 31 more than the team managed across its day-and-a-half of activity in the first week.

"After the last couple of weeks it's a huge relief to get a day like today," said Russell, who finished ninth-fastest, 1.9s off the pace.

"We've learned so much about the car at lower fuel and higher fuel, it gives us such a good indication moving forward into the next few days, and for the guys back at the factory who are already designing the upgrades."

Russell gave the FW42 its first on-track run last week but racked up just 40 laps over the next day, while teammate Robert Kubica managed 48.

It meant Williams's appearance in week one was little more than a glorified shakedown, but Russell said the point was to make sure it could hit the ground running this week.

He described it as a "proper day one" for the team and said he can start to feel the car's potential.

"It certainly hasn't made up for last week because everyone else at this stage is fine-tuning the balance, whereas we're needing to understand the car," said Russell.

"But I feel we're in a good spot. The car isn't in a perfect window right now but I feel like it's definitely got the potential."

Williams ran its car in a compromised state last week while it completed its initial runs, although Russell said it was "probably 95%" of what it is running now.

He hailed the effort that the team has put in after the delays and said the reward for everybody putting "their heart and soul into make sure today ran smoothly" was Williams being able to complete its Tuesday running exactly as planned.

"I think everybody's happy with today's performance – not in terms of lap times but in terms of no issues," he said.

"Everybody's working so hard. We're all human. Everybody is pretty knackered. They've been performing 24/7 over the last two weeks to get us ready."

In terms of performance, Russell said the balance was "not correct" on Pirelli's softer compounds but reckoned that was "only normal" so early in the team's programme.

"We're still finding the limitations, what the different compounds do," he said.

"There was no reason to worry we didn't find the lap time we should have done, because that should come.

"There's definitely room for improvement on the softer compounds but we've only ran two sets of C3, one set of C4 and one C5 with lower fuel. We're still learning."