Williams' George Russell completed 119 trouble-free laps of the Circuit de Catalunya on Tuesday, the young rookie admitting the mileage was a big relief following last week's drama.
Williams entered pre-season testing on the back foot because of aproduction and build-up delay that impacted its Fw42's timely arrival in Barcelona.
However, without any significant reliability issues to contend with, Russell was able to diligently run through the Grove-based outfit's data gathering and performance programmes.
"After the last couple of weeks it's a huge relief to get a day like today," said Russell, who clocked in at the end of the day with the ninth fastest lap.
"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."
Last week's late start on Wednesday was followed by two laborious sessions that were more akin to shakedown runs than genuine work sessions.
Tuesday was therefore seen as the team's first proper day of running.
"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.
"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."
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