Mercedes Nico Rosberg took the top spot on the opening day of the second pre-season test in Circuit de Catalunya, Barcelona, Spain on Tuesday.
The Mercedes driver managed to clock 1:22.616 a lap, just seven thousandths quicker than Kimi Raikkonen in the Lotus.
Both times came on the medium compound tyre toward the end of the session after Sebastian Vettel had led for much of the morning, before dropping to fourth behind Fernando Alonso.
The Spaniard made the most of his first day behind the wheel of the F138 by posting 110 laps, almost 30 more than his nearest rival.
Pastor Maldonado who debuted the new Williams FW35 for the first time went fifth fastest with a lap just over 1.1 seconds slower than Rosberg.
The session saw two red flags thanks to Esteban Gutierrez' s low fuel run, which saw him stop on circuit. Meanwhile Charles Pic stopped with a mechanical issue. He finished 11th behind Max Chilton in the Marussia.