Flying Finn Valtteri Bottas set the fastest time of testing so far this pre-season whilst compatriot Kimi Raikkonen's day was ended early by a crash in Barcelona on Wednesday.
Bottas, embarking on his debut season with Mercedes after taking over from retired world champion Nico Rosberg, bettered his previous best time from last week by nearly four-tenths of a second in 1min 19.310 as Mercedes reasserted their dominance at the top of the timesheets.
Team-mate Lewis Hamilton took over from Bottas in the Mercedes for the afternoon, but was only sixth-fastest as he concentrated on longer runs in completing 79 laps.
"We're definitely making good progress as a team and I'm feeling more and more comfortable in the car every time I go out on track," said Bottas.
"There's still plenty of work to do so we need to maximise the next two days of testing.
"Fastest laps don't matter in testing. What matters is where you are when you get to Melbourne (season-opener on March 26)."
Ferrari's Raikkonen was third-fastest but missed out on most of the afternoon after a prang with a tyre wall after running wide at turn three.
The 2007 world champion was unhurt but his stricken SF70H had to be towed away and was unable to run for the rest of the day.
Felipe Massa was surprisingly fastest in his Williams on Tuesday and the Brazilian posted another promising time just over a tenth of a second slower than Bottas in second fastest.
Dutch wonder kid Max Verstappen also suffered a late breakdown, but only after he had completed over 100 laps in the fourth-fastest time in his Red Bull.
In a familiar tale of the first six days of testing, two-time world champion Fernando Alonso endured another frustrating time in the beleaguered McLaren MCL32.
The Spaniard lagged 12th out of 14 drivers in action after posting just 46 laps in a series of truncated outings.