Valtteri Bottas clocked the fastest time of the week as the opening session of pre-season Formula One testing ended on Friday giving world champions Mercedes an early psychological edge and leaving rivals mystified by their revolutionary steering device.

Bottas, the 2019 runner-up to six-time champion and teammate Lewis Hamilton, timed 1min 15.732sec off a total of 65 laps.

Hamilton, who had been fastest on Wednesday's opening day, was second on Friday, also dipping below the 1min 17sec mark.

Although pre-season testing is no guarantee of what will eventually play out in the 21-race season, Mercedes will be delighted in already having the likes of Ferrari and Red Bull pondering the impact of their DAS (Dual Axis Steering).

The device, which was spotted in Hamilton's car on Thursday and used by Bottas on Friday, allows the driver to pull the steering wheel towards him on the straights and push it back on the corners.

On top: Mercedes' Valtteri Bottas set the pace on Friday
On top: Mercedes' Valtteri Bottas set the pace on Friday AFP / LLUIS GENE

Its purpose is to improve grip and stability.

Bottas said Friday that the DAS has been in development for a year and was used for the first time on track in Barcelona after being tested in a simulator.

It could give the German outfit a decisive advantage before a change of regulations which come into effect in 2021.

"What we've learnt from running with that now is, yes, we think we are happy to have it in the car and we wouldn't run anything like this if we didn't think there would be a performance gain," the Finn told Sky Sports.

"We're still learning about it, in which kinds of sessions, areas, conditions the gains are actually going to be. And if it's going to be in qualifying, in the race etc.

Plenty to think about: Ferrari's driver Sebastian Vettel
Plenty to think about: Ferrari's driver Sebastian Vettel AFP / LLUIS GENE

"For us it was a pretty big project, and it would be a pretty difficult to copy. I hope we can have an edge with that."

Four-time world champion Sebastian Vettel, who missed the first day of testing through illness, saw his morning session cut short when his Ferrari broke down.

He eventually totalled 100 laps for the day but finished in 12th place in the time charts with a best of 1:18.384.

Ferrari team chief Mattia Binotto shrugged off the team's troubles, insisting that testing was mostly an opportunity to experiment with set-ups.

"We have changed our approach for these tests," he said.

"We are taking advantage of the first week to configure the car mechanically and aerodynamically and optimise the settings.

"It's true that we were more optimistic at the same time last year because it was easier to go fast but we did not focus as much on the set-up."

The second session of pre-season testing takes place at the same Montmelo circuit from Wednesday-Friday next week ahead of the 2020 world championship opener at Melbourne on March 15.

Times

Valtteri Bottas (FIN/Mercedes) 1:15.732 (65 laps), Lewis Hamilton (GBR/Mercedes) 1:16.516 (73), Esteban Ocon (FRA/Renault) 1:17.102 (76), Lance Stroll (CAN/Racing Point) 1:17.338 (116), Daniil Kvyat (RUS/Alpha Tauri-Honda) 1:17.427 (62), Antonio Giovinazzi (ITA/Alfa Romeo Racing-Ferrari) 1:17.469 (152), Daniel Ricciardo (AUS/Renault) 1:17.574, Max Verstappen (NED/Red Bull-Honda) 1:17.636 (86), Pierre Gasly (FRA/Alpha Tauri-Honda) 1:17.783 (59), Alexander Albon (THA/Red Bull-Honda) 1:18.154 (83), Carlos Sainz (ESP/McLaren-Renault) 1:18.274 (76), Romain Grosjean (FRA/Haas-Ferrari) 1:18.380 (48), Sebastian Vettel (GER/Ferrari) 1:18.384 (100), Lando Norris (GBR/McLaren-Renault) 1:18.454 (49), Nicholas Latifi (CAN/Williams-Mercedes) 1:19.004 (72), Kevin Magnussen (DEN/Haas-Ferrari) 1:19.709 (4)