Mercedes set the pace in practice for the Spanish Grand Prix, with Valtteri Bottas and Lewis Hamilton finishing with the fastest times on Friday.
Bottas led the first session and Hamilton the second at the Circuit de Barcelona-Catalunya, both times without having to use the faster supersoft tires.
Sebastian Vettel, who trails Hamilton by four points in Formula One, kept Ferrari close in the morning session, but Red Bull was the main challenger to Mercedes in the afternoon. Daniel Ricciardo was second and Max Verstappen third.
Bottas was a tenth of a second faster in the morning session than Hamilton's afternoon time at the 4.6-kilometer (2.8-mile) track outside Barcelona.
Most teams brought significant updates for their cars to Spain as F1 returned to Europe after races in Australia, Bahrain, China and Azerbaijan. Mercedes and Red Bull appeared to have improved the most, although Ferrari usually shows its best pace in qualifying and in the race.
Ricciardo, who crashed with Verstappen in Azerbaijan, had a tough start to his weekend in Barcelona by hitting a tire barrier midway through the first session, losing a lot of track time.
Kimi Raikkonen, in the other Ferrari, was sixth fastest in the afternoon and had to retire late because of an apparent engine problem.
Crowd-favorite Fernando Alonso was sixth fastest with McLaren in the morning but was 12th in the second session.
Robert Kubica, Williams' reserve driver, drove in an official F1 session for the first time since almost losing a hand in a rally crash in 2011. He participated in preseason testing this year.
The team had to make some modifications to Kubica's car because the Polish driver doesn't have full movement of his right hand.
Kubica finished with the second-to-last time in the first session, in front of teammate Lance Stroll. Sergei Sirotkin was back in the car for the afternoon run.
"In the end it's difficult to say that it was enjoyable because our car balance was very bad and it was very difficult to drive, so it was difficult to enjoy," Kubica said. "But I'm satisfied with the session and with how I reacted to difficult conditions. It sounds strange that you can be happy with P19, but actually I'm happy."
Stroll nearly hit the tire barriers after going off the track with about a half-hour left in the first session.
Many other drivers also struggled to keep their cars on the track.
Brendon Hartley of Toro Rosso and Romain Grosjean of Haas were among those who went into the gravel in the morning. Bottas and Vettel also spun but were able to continue their runs without significant damage.
Romain Grosjean, who crashed under the safety car in Azerbaijan, was the first to face trouble in the afternoon session, losing control of his Haas and getting stuck on the gravel. Verstappen also went off the track, as did Raikkonen shortly before enduring the mechanical problem.
Sergio Perez had a mechanical issue with Force India and had to retire near the end of the second session.