Carlos Sainz kept McLaren at the top of the timesheet on Wednesday morning in Barcelona, but the session delivered another set-back to Ferrari following a heavy crash for Sebastian Vettel.
Sainz was the only driver to lay down a sub-1m18s lap in the morning session, the Spaniard setting the pace and edging Vettel, by over one second, and Red Bull's Max Verstappen.
Pre-season test Day 5 - Morning session
For all the disruption caused by the elements last year, Barcelona's weather has so far been a great benefit to teams during pre-season testing, with the sun shining bright once again on over the Circuit de Catalunya Wednesday morning.
After yesterday's interesting session, marked by limited running for Mercedes and Ferrari and Red Bull and McLaren's outright performances, everyone was keen to get back out on track and resume their programmes.
Valtteri Bottas was looking to make up for Tuesday's lost mileage, so the Finn had his head down at the outset.
Max Verstappen, picking up where Gasly had left off for Red Bull the day before, set the early pace, establishing a sub-1m19s benchmark time in the first 45 minutes of the session.
But over at McLaren, Carlos Sainz immediately followed his team mate's lead from Tuesday, popping to the top of the time-sheet.
The Spaniard didn't let up after his initial flyer however and lowered his time thanks to two purple sectors - and a set of 'supersoft' C4 tyres - to lay down a 1m17.503 marker that was McLaren's fastest lap this winter and the second best time overall, behind Nico Hulkenberg's 1m17.393s.
Ninety minutes into the morning, grinding around the Circuit de Catalunya, Sebastian Vettel steadily improved to place himself second behind Sainz, with 27 laps on the board, almost as much as the Scuderia achieved during the whole of yesterday.
In a repeat of Tuesday's woes, McLaren's MCL34 ground to a halt after Sainz's fast stint, the Spaniard stopping shortly after exiting the pits, with the team later diagnosing a relatively benign water pressure issue.
Shortly after, the red flag was brought out a second time when Vettel crashed heavily at Turn 3, the victim of an apparent "mechanical problem".
The German driver was transported off to the medical center for checks but eventually given a clean bill of health. However, the incident was another set-back for the Scuderia after yesterday's delay.
It also meant a lengthy interruption of the session as the Circuit de Catalunya marshalls repaired the barriers at Turn 3.
Testing resumed with 70 minutes left on the board. At that point, Max Verstappen led the lap count from Valtteri Bottas and Daniel Ricciardo, the Renault camp running without a glitch through its workload, privileging valuable data gathering over a quest for speed.
Behind the wheel of Williams' FW42, Robert Kubica added another "proper day" of running to his team's schedule, continuing the British outfit's catch-up programme unhindered.
Over at McLaren it was a quick turnaround for Sainz who was up and running once again with 30 minutes remaining on the clock before the daily lunch-break. Ferrari's morning however was over.
Verstappen reigned in the speed but piled on more laps on to his tally, the Red Bull driver conducting longer runs, with Honda's unit still performing flawlessly.
Both Alfa Romeo and Racing Point enjoyed a quiet morning, but Kimi Raikkonen picked up the pace to move ahead of Sergio Perez with the sixth fastest lap.
Bottas also racked up strong mileage as the team continued the development of its revised W10.
As lunch beckoned, McLaren's Carlos Sainz headed to a well-deserved dish of paella as the fastest man of the morning!