Lewis Hamilton made an encouraging start to testing by setting the pace at the Circuit de Barcelona-Catalunya in his new W08 on Monday.
The three-time Formula One world champion's quickest lap was a second faster than he managed in pre-season testing in Spain last year, with Sebastian Vettel second-fastest.
Hamilton clocked one minute 21.765 seconds on soft-compound tyres as Mercedes racked up 152 laps on day one.
"The car looks fantastic and it feels great," said Hamilton. "We did lots of laps and collected loads of information, so we can try to improve the car as we move forward."
Ferrari's Vettel was just a 10th of a second slower than Brit Hamilton, who is the favourite to regain the world title after finishing runner-up to retired former team-mate Nico Rosberg last year.
Felipe Massa was third on the timesheets for Williams ahead of the Haas of Kevin Magnussen, with Hamilton's new team-mate Valtteri Bottas sixth.
Red Bull's Daniel Ricciardo endured sensor and battery issues with his RB13 and could only manage 50 laps, but the Australia said there is "a lot more to come" from.