SOCHI, Russia (AP) Lewis Hamilton set the fastest time in Friday's second practice session for the Russian Grand Prix, while Sebastian Vettel stopped his Ferrari on the track and faces a grid penalty.
Ahead of qualifying on Saturday, both practice sessions were topped by Mercedes, underlining the dominance of a team which has been on pole for 39 of the last 41 races.
Hamilton edged Vettel by .652 seconds in the second session, and set the fastest time over the two sessions. A longer run showed good indications for race pace, he said.
''It seemed to go well, seemed to be quite smooth and generally quite good pace, so I'm quite happy with it,'' Hamilton said.
His teammate and the championship leader, Nico Rosberg, was top in the first session but third in the second, .867 seconds behind Vettel. Rosberg has won all three races, and said Mercedes was adapting well to the smooth, rarely used asphalt in Russia.
''It's different to everywhere else,'' he said. ''It's not easy to get it right, car-wise and driving-wise, you know, so it's been an interesting day but good. We got off to a good start, all of us, so I'm pleased with that.''
Vettel was fast early on but came to a halt on the start-finish straight just before the halfway point of the 90-minute session, saying over the team radio he ''lost electronics.''
Ferrari later said it would change his gearbox, which will earn Vettel a five-place grid penalty for Sunday's race. The change was due to collision damage from the last race in China and unrelated to the electronics problem, the team said.
Despite losing valuable running time to the electronics error, Vettel said after practice that the car was in good shape.
''The feeling was alright, the balance wasn't yet where I want it to be, but we can improve,'' he said, adding that Sochi's Olympic Park was ''a circuit which should come our way.''
Near the end of the second session, Manor's Pascal Wehrlein stopped on the track, too, telling his team he suffered a power failure.
In the first session, Rosberg led Hamilton by .722 seconds. Vettel was third fastest, trailing Rosberg by 1.048, with his teammate Kimi Raikkonen in fourth, and the Williams of Felipe Massa in fifth.
Red Bull's Daniel Ricciardo did a brief stint at the start with the team's ''aeroscreen'' concept, which works like a reinforced windshield.
The team released video of the screen deflecting a wheel at high speed, and also a one-kilogram metal projectile similar in size to the metal spring which left Massa with a fractured skull in 2009, when he was driving for Ferrari.
The screen is a rival to a metal ''halo'' tested by Ferrari last month. Both concepts have come in for criticism over the way they look.
The series is looking at ways of improving cockpit security to leave drivers less exposed to the risk of head injuries after French driver Jules Bianchi and British IndyCar driver Justin Wilson died last year.
Two test drivers took part in the first session: Russian Sergei Sirotkin was 14th for Renault and 19-year-old Mexican Alfonso Celis 22nd and last for Force India.
There were some struggles for the American Haas team, which stunned the F1 world when Romain Grosjean was sixth and fifth in the team's first two races but made little impact at the last race in China.
On Friday, Haas' best result was 16th in each session and principal race engineer Ayao Komatsu said the team was having difficulties getting the best out of the tires.
''Every day we're finding some new problems, areas we need to improve,'' he said. ''So it's just a matter of prioritizing them because we haven't got many people, so we need to be realistic.''