Having continued with its policy of alternating its drivers, Nico Rosberg was on duty for the German team this morning before handing over to teammate Lewis Hamilton.
And while the sport's powers-that-be will have collectively groaned when Rosberg, trying the yellow-banded soft tyres for the first time this year, went quickest, closing to within 0.2s of Sebastian Vettel's best - set last week on the new ultrasofts - those same powers-that-be must have allowed themselves a wry smile when the world champion ground to a halt halfway down the pitlane.
As we wondered whether Mercedes had finally encountered a problem, pondered the thought of the W07 developing an Achilles heel, the issue - whatever it may have been - was quickly rectified and moments later Hamilton was posting his first time of the day.
Indeed, the Briton went on to complete a race simulation, eventually adding a further 90 laps to the 82 Rosberg completed this morning.
Fact is, other than a couple of issues with the wheel-guns during the race-sim pit stops, the W07 ran as faultlessly as ever, which is more than can be said for some of the opposition.
Rio Haryanto was side-lined for much of the morning with an oil leak, and when the hapless Manor driver finally got going, in the moments before the lunchbreak, out came the red flags when Fernando Alonso ground to a halt at the end of the pitlane, the result, it was subsequently discovered, of a "small telemetry glitch".
It was a difficult day for Haas also, a "fuel system issue" bringing Esteban Gutierrez' day to an early close, the Mexican having completed just 23 laps.
Kimi Raikkonen was the cause of another red flag, the Finn stopping at T1 with a gearbox problem, while the final red flag of the day came in the final fifteen minutes of the session, Kevin Magnussen stopping his Renault at T3.
Another driver who lost a lot of track time was Daniil Kvyat, the Russian needing his crew to resort to the fire extinguishers when he returned to the pits with what was thought to be a rear brake fire. However, it is understood the problem was actually the 'cake tin' on the rear brakes.
Valtteri Bottas became the latest driver to try the ultrasoft rubber, posting the second best time in the process, but it was noticeable that when the purple banded tyres go off, they go off very, very quickly.
Despite his telemetry glitch, it wasn't a bad day for Fernando Alonso, the Spaniard finishing third on the timesheets and completing a respectable 93 laps.
With each day, the Toro Rosso, now sporting its new (?!?) livery, looks better and better, and as Max Verstappen reeled off the laps - 144 in total - one wondered if the Faenza team might beat its sister outfit this season, and should that be the case how it will go down with Mr Mateschitz and pals.
Hulkenberg, Nasr and Magnussen each completed over 100 laps, a highly impressive feat, particularly for the Sauber which was making its track debut today.
Nico Rosberg emerged for his first run at 09:00, completing 82 laps before the hour-long lunch break following which Lewis Hamilton completed a further 90 laps in the afternoon.
Today's programme focused on lower fuel running during the morning - including running soft tyres for the first time during the winter - and a race simulation in the afternoon.
"It was a decent morning and we're making progress," said the German. "It was nice for me to be able to push some more and produce some stronger lap times. We were doing qualifying practice this morning - it's important to practise that as well. And that's when it's all the more enjoyable; taking the car to the limit and really pushing. It's been a good test, reliability has been good, the speed has been there - it's all working to plan."
"It was a great afternoon of running with great mileage," added Hamilton. "We did a race simulation and after that I feel I'm ready for race one. The car is feeling great and was consistent all day. It's been a monumental effort from this team to get the car to where it is today. I'm truly grateful to everyone for every bit of work they have put in so far."
"It was a good day today," said Bottas. "We managed to run without any major issues with the car. It was very interesting discovering more about the FW38 and what kind of set-ups work for this car. I think we managed to make progress. We did some short runs today which I really enjoyed.
"I still agree with what I said last week," he concluded, "we have improved in the areas where we were weak last year. I am looking forward to tomorrow and continuing testing.
"Today was a very good day for us," added Rod Nelson, the Grove outfit's Chief Test & Support Engineer. "The car ran faultlessly. We did lots of quite significant tests throughout the day as well as a bit of a qualifying simulation. We managed to get through our planned programme without any major problems, so overall we are really pleased."
Fernando Alonso returned to the cockpit for McLaren as the team focussed on bedding in a new power unit, which had been slightly updated from last week's to move closer to the specification scheduled to run in Melbourne. Much of the day's programme concentrated on engine mode and driveability tests, tyre compound assessments as well as aerodynamic set-up changes.
A technical glitch halted running just before the lunch break, when an electronics issue caused the car to lose telemetry data. This was identified and reset over lunch and Alonso returned to the track shortly after 15:00.
Despite some niggles in the afternoon as the new power unit bedded into the chassis, today's run programme was concluded successfully, and the Spaniard completed a total of 93 laps.
"After the disappointment of not being able to complete much running at the end of last week's test, it was positive to get so many laps on the board today," admitted Alonso. "We tested a number of different configurations and spent a lot of time working on the power unit and collecting data from the aerodynamics. It's too early to tell a real difference compared to the engine last week, as we focussed today on shorter runs and adjusting the balance and set-up of the car.
"However, today's full day of running has been a good start to the second test, and has provided us with lots of useful data that we can analyse overnight before Jenson continues the programme tomorrow."
"After a busy and long weekend for the whole team out here at the track, back at MTC and in Sakura, we started the second test off on a positive note," added Director of Engineering, Matt Morris. "It's always encouraging to get some good mileage under our belt on the first day.
"We spent a good chunk of the day evaluating the power unit functionality, although our attention hasn't yet turned to assessing outright performance. The important thing was to get as many miles on this package as possible, so from that point of view and the amount of data we have collected today, it has been very positive.
"We will continue to maximise our track time over the coming days, to work on ironing out reliability niggles and move on to longer runs. We also have some new aero parts arriving overnight, which will be assessed on track tomorrow with Jenson."
"We had an unexpected control electronics shutdown just before lunch time," added Yusuke Hasegawa, Honda R&D Co Ltd Head of F1 Project & Executive Chief Engineer, "believed to be the cause of a software problem. Apart from that, we were able to run a decent amount of laps to finish the first day of the second test in Barcelona.
"On the power unit side, we tested many different modes of running to prepare for the race, so it was a busy day for the system engineers. As a package, we were able to test different balances of the car and tyres together with the power unit.
"We are looking to perform further detailed checks on the power unit during tomorrow's testing."
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