Nico Rosberg beats Lewis Hamilton in Spain practice

Rosberg comes into the grand prix 27 points behind Lewis Hamilton in the drivers' standings

Nico Rosberg pipped Mercedes team-mate Lewis Hamilton to set the pace in Spanish Grand Prix first practice.

The German headed Hamilton by just 0.07 seconds as the two silver cars were a second clear of the Ferraris of Sebastian Vettel and Kimi Raikkonen.

Hamilton appeared to have a faster lap in him, but after setting two quicker sector times he backed off following a slide in the final chicane.

Susie Wolff was 14th on her first outing of the season for Williams.

The Scot was six places and 0.8secs behind team-mate and race driver Felipe Massa.

Wolff was the slowest of the three stand-in drivers running during the session, with Sauber's Rafaelle Marciello 12th and Lotus's Jolyon Palmer 13th.

Marciello was two places behind Sauber race driver Felipe Nasr in 10th, while Palmer headed Pastor Maldonado, who was down in 18th after doing just one run.

Williams reserve driver Susie Wolff took part in her first practice race of 2015, finishing 14th

Ferrari came to Spain hoping to close the gap to Mercedes and have a major upgrade on their car, with many of the aerodynamic surfaces reshaped to create more downforce.

However, Mercedes appear so far to have retained their advantage in overall downforce judging from the margin between the two cars.

Rosberg was 0.978secs quicker than Vettel, who headed Raikkonen by just 0.028secs.

Barcelona is a severe test of the overall aerodynamic performance of an F1 car because of its series of long-duration corners of varying speeds, which reward consistent downforce.

So when a car has an advantage here it tends to be a reflection of its aerodynamic qualities, assuming the engines are of relatively similar performance, as is the case this year with the Mercedes and Ferrari.

Behind the big two, Toro Rosso drivers Carlos Sainz Jr and Max Verstappen were fifth and sixth.

That put them ahead of senior team Red Bull, for whom Daniil Kvyat was seventh fastest as the team tried out a new shorter nose design on the car.

The Russian did only one run, while team-mate Daniel Ricciardo did not get out until the final 10 minutes of the session because of engine reliability problems but still managed ninth fastest.

The McLaren-Hondas of Fernando Alonso and Jenson Button were only 15th and 16th.

The team had been expecting a step forward in performance as a result of a series of upgrades on car and engine but it was not reflected in the times in the first session.

One of the developments was a new front wing, and Button was heard complaining on the radio that the "balance was completely wrong".

There was only 0.004secs between the two drivers, with Alonso just ahead. The Spaniard ended the session with a dramatic spin on the exit of Turn 11 into Turn 12.