Sergio Perez put McLaren on top of the pile as the second F1 test of the off-season reached the half way stage at Catalunya.
The Mexican, on his final day in the car before handing over to Jenson Button, set the best time of the week to date as he became the first man to break the 1min 22secs barrier.
Perez's quick time came during a one-lap run on the soft compound Pirelli tyre in the afternoon session, with a 1min 21.848secs lap putting him three tenths of a second clear of defending champion Sebastian Vettel.
The German had led the way for Red Bull during the morning session before switching his attention to a race run in the afternoon, although he spent a number of spells in the garage before his car came to a halt on the pit exit to bring out the first of two red flags.
“We had some issues with reliability today and we couldn't do as many laps as we wanted, but I think it's better that this kind of thing happens today rather than in Australia,” the defending champion said. “It was nothing disastrous, just small things that hampered us today. We stopped the car once as we saw a small loss of pressure. The second stop towards the end of the day caused a red flag and it simply took too much time to turn everything around, so we couldn't get out again.”
Despite that, Vettel maintained second spot ahead of the Lotus of Kimi Raikkonen; the Finn being another driver to endure a troubled day on track.
Raikkonen had hoped to see more track-time after telemetry issues on Tuesday but a gearbox problem meant he sat out most of the morning session watching on the from the sidelines as the team changed the part.
Once on track however, Raikkonen was soon up to speed and managed to edge out the Mercedes of Lewis Hamilton to secure a place in the top three.
"We had a good day today and it was nice to get so many laps in," Hamilton said of his first day of running this week. "We're making continual steps forward in terms of our understanding of the set-up and the tyres. The hard compound work in particular was pretty good today, although all of tyres are showing degradation so looking after them isn't easy.
"The guys are working really hard both here and at the factory, and it's encouraging to see the amount of work that's being put in. We're focusing on our own programme and just getting on with it."