Barcelona - Ferrari's Sebastian Vettel took advantage of a new ultrasoft tyre to propel himself to the top of the timesheets for the second consecutive day of pre-season testing in Barcelona.
Vettel's speed has heightened expectations that Ferrari can challenge Mercedes' dominance of the sport for the past two years.
"The car is a step forward, so that is positive, and the power unit has improved," said the German.
"We know there are areas we still need to improve, but overall I am happy."
The four-time world champion was also one of eight drivers to break the 100-lap mark as a lack of off-season technical changes resulted in much better reliability than normally expected so early in the season.
Red Bull's Daniel Ricciardo also used tyre manufacturer Pirelli's fastest tyre to get second fastest, but was seven tenths down on Vettel's flying lap of 1min 22:810sec.
Sergio Perez in the Force India was just a further tenth of a second back in third fastest.
However, once again Mercedes were arguably the most impressive performers as Nico Rosberg completed a marathon 172 laps to outdo teammate and defending world champion Lewis Hamilton's 156-lap run on Monday.
Rosberg was also nearly half a second faster than Hamilton and went within 0.2 seconds of his qualifying lap to land pole position at the same circuit for last season's Spanish Grand Prix with the quickest time on the slower medium tyres.
The McLaren-Honda partnership had another busy morning off the track with the news Honda chief motorsport officer Yasuhisa Arai will be replaced by Yusuke Hasegawa before the first race of the season next month.
On the track, there was more significant progress in their reliability as Fernando Alonso clocked up more laps in one day than he managed in the whole of testing last season with 119.
However, the MP4-31 was still well short on speed as the two-time world champion was only ninth fastest ahead of just Renault rookie Jolyon Palmer and Max Verstappen's Toro Rosso.
Renault have returned as a constructor this season having taken over the financially stricken Lotus, but have struggled most early on as the only team to have failed to run a century of laps combined over the opening two days.
There was more good progress for F1 newbies Haas as Esteban Gutierrez was sixth fastest.
Williams were clearly the best of the rest behind Mercedes and Ferrari last season, but their potential for the upcoming campaign remains hard to judge as Valtteri Bottas completed a trouble-free 134 laps, but was only down in seventh fastest.