Mercedes, with Valtteri Bottas again quicker than world champion Lewis Hamilton, led the way in the first Spanish Grand Prix practice on Friday with Ferrari's Sebastian Vettel the best of the rest.
Bottas put down an impressive marker with a best lap of one minute 18.148 seconds on the soft tyres, 0.849 faster than his team mate who won from pole last year.
Vettel, who will be chasing his fourth successive pole position on Saturday, was on the quicker supersofts but 0.950 slower than the Finn around a sunny but windy Circuit de Catalunya.
Hamilton leads the German, also a four-times world champion, by four points after four races but owes his advantage to a healthy slice of luck.
"At the moment I am punching below my weight, and that is not sufficient to win a world championship," the Briton told reporters on Thursday.
"I have zero comfortability... I'll take it for now but in the long term l can't continue to rely on it."
Red Bull's Max Verstappen was fourth fastest but Australian team mate Daniel Ricciardo crashed in his first track session since the two drivers collided in the Azerbaijan Grand Prix 12 days ago.
The virtual safety car was deployed briefly, with Ricciardo's car running across the gravel and into the barriers, with several others also spinning. The Australian was classified seventh.
"Thankfully the damage is pretty light," team principal Christian Horner told Sky Sports television. "It was the old front wing so he's a lucky boy."
Home hero Fernando Alonso started the weekend a morale-boosting sixth, behind Ferrari's Kimi Raikkonen, with his McLaren displaying a heavily-revised three-pronged nose -- one of several conspicuous upgrades around the pit lane.
Ferrari caught the eye with wing mirrors mounted on the halo head protection device.
Williams had another trying session as Polish reserve Robert Kubica took part in a race weekend for the first time since 2010, three months before he suffered serious injuries in a rally crash.
The former race winner was 19th, in 1:21.510, and faster than teenage Canadian team mate Lance Stroll in last place on the timesheets with a best of 1:22.756 after his car went off into the gravel.
Kubica's car had a newer floor than Stroll's, however.
"Unbelievable how bad the balance is," Stroll complained over the radio.
Kubica was replacing Russian rookie Sergey Sirotkin, who returns for second practice and the race but has a three-place grid penalty.
"It's a good feeling, for sure, being a part of a race weekend after seven and a half years," said the Pole.