Jerez de la Frontera - Brazilian rookie Felipe Nasr upstaged former world champions Kimi Raikkonen and Fernando Alonso as he went fastest on the third day of pre-season testing in Jerez.
Nasr's Sauber C34 car has been an early season hit and his best time of 1min 21.545 in 108 laps was enough to end Ferrari's dominance on the leaderboard from the opening couple of days.
However, technical problems persisted for McLaren and Red Bull as Alonso and Daniel Ricciardo were at the back of the grid both in time and laps completed.
"It is only testing, it doesn't mean anything to be leading the testing today because I don't know what the others are doing with fuel," said Nasr.
"But so far it's so positive and much better than I expected."
Raikkonen took over from new teammate Sebastian Vettel in the SF15-T, but couldn't match the pace set by the German on the opening two days as he was 0.205sec behind Nasr in his 92 laps.
"I think the whole team has done a very good job over the winter after last year when we had difficult times," said the Finn.
"It was a positive day, a lot of improvements. It is a good starting point."
Double world champions Mercedes continued to prioritise mileage over speed as Nico Rosberg racked up 151 laps to add to his world record 157 lap effort on day one.
Rosberg could even have managed more, but his W06 had to be towed back to the garage after stopping on the track midway through the day.
McLaren looked to have put a disappointing opening couple of days during which Alonso and Jenson Button managed just six laps apiece behind them early in the day.
Alonso was able to log 32 laps on Tuesday morning, albeit 14 seconds off the pace set by Nasr.
Yet, worse was to come as cooling pressure problem in the power unit prevented any running after lunch to once again leave them as the team with the least amount of mileage on the day.
The Spaniard will now have to wait until the second test in Barcelona later this month to get back in the Mp4-30 as Button will take over for Wednesday's final day in Jerez.
"This morning it was just a relief to see the car get out of the garage and nine o'clock and running faultlessly during a few hours," said McLaren's racing director Eric Boullier.
"We could have run maybe half an hour at the end, but instead we decided to wait and make sure it's right for tomorrow."
Barcelona will also be a big test for the only non-Mercedes driver to win a grand prix last season as Ricciardo has been restricted to just 83 laps over two days in Jerez.
"Obviously we were a bit limited with our running today, the last hour was a bit better. We got some long runs done, but I think testing is where you get to know the car a bit better and feel how it uses the tyres," said the Australian.
"It's always tricky to rack up a lot of laps at the first test. Obviously some teams have done it and I think we've got a few more opportunities in Barcelona."
Red Bull's unreliability is even more alarming as another rookie Carlos Sainz gave the Spanish fans more to see than the home hero Alonso as he completed 136 laps for their junior team Toro Rosso.