Felipe Massa set a blistering pace for Williams on day one of the final pre-season test in Barcelona.
The Brazilian smashed the fastest time seen so far in Catalonia with a 1:23.500 on the soft compound tyres during a qualifying simulation to lead by over a second from Lewis Hamilton as the defending world champions had their worst day of pre-season so far.
Massa also proved that while Mercedes' pace on the medium compound was impressive, the 1:24's were also achievable for other teams as he set the fastest time of the day, a 1:24.861, for a car using the white-marked Pirellis.
Mercedes hit by hybrid problem
As mentioned, Mercedes had a difficult day on Thursday. Hamilton initially set the timing screens alight with his best of 1:24.881 but light rain just before lunch curtailed running in the morning before an ERS problem kept the W06 stranded in the garage throughout the afternoon.
That meant that while Mercedes surpassed 1,000 laps over the nine days of testing so far, their total of 48 laps was the lowest they had achieved so far this year.
Sauber lead lap count
It was a relatively quiet day across the grid as teams tested new aerodynamic and engine parts for the first time as the serious business of preparation for Melbourne began.
Only two teams surpassed the 100-lap mark on the day with Felipe Massa running regular medium-length stints but it was Marcus Ericsson in the Sauber who had the highest total on the day with 122 laps completed including a full race simulation.
The Swede also completed two low-fuel runs on the super-soft tyres ending with the second best time of 1:24.267 albeit still slower than Romain Grosjean's 1:24.0 on the same tyres from the final day of the previous test.
Teams test Melbourne upgrades
Daniil Kvyat had a shaky start causing a brief red flag as he stopped at the exit of the pit-lane but would eventually have a solid day in the Red Bull - testing new updates to the Renault power unit - and completed 75 laps with the fourth best time albeit 2.5 seconds slower than Massa's benchmark.
Ferrari too had a similar theme before concentrating on race simulations later in the week but with Kimi Raikkonen behind the wheel, the Finn managed 80 laps setting the sixth best time.
The team with perhaps the most upgrades at the final test is Toro Rosso with new wings, bodywork, front nose and rear suspension.
Despite all the new parts, and despite causing a red flag in the afternoon, Carlos Sainz still managed a third best tally of 86 laps on the day. Onlookers suggested the new STR10 was one of the least impressive cars to watch on track and perhaps that was reflected in the slowest representative lap time of the day.
In a day of low profiles, Lotus were perhaps one of the least notable teams on the grid but with 75 laps and the fifth best time it was still a productive day for Romain Grosjean on just his third day in the car this year.
Finally at McLaren, it was another troubled day for the new Honda partnership.
McLaren struggles continue
Jenson Button managed just seven laps in the morning with only one multi-lap run before a hydraulic leak meant the team are having to replace the power unit to allow Kevin Magnussen, standing in for Fernando Alonso, to run on Friday.
The 2009 champion managed just one timed lap, a 1:31.479, to be eight seconds off the fastest time.
Williams keep Mercedes on their toes
A largely subdued day but also one that proves Mercedes are still not guaranteed to be the runaway leaders that perhaps we thought the could be following their impressive pace towards the end of the second test.
With most teams managing to complete their programmes on Thursday we can expect a lot more running so long as the forecast rain holds off on Friday.