Nico Rosberg comfortably outpaced Lewis Hamilton to head first practice at the Russian Grand Prix.
The Mercedes team-mates led Sebastian Vettel and Kimi Raikkonen of Ferrari, with Williams' Felipe Massa fifth and Red Bull's Daniel Ricciardo sixth.
Hamilton has said he cannot afford to drop any more points to Rosberg, who has a 36-point lead in the title.
The session also saw Red Bull run their version of cockpit head protection on Ricciardo's car for one lap.
The 'aeroscreen' is the design the former champions believe is preferable to the 'halo' which was tried by Ferrari in pre-season testing.
Both devices feature a similar elliptical structure to protect the driver's head from impact from large airborne debris but differ in how they are mounted to the car and their appearance.
Drivers called for the device to be introduced by next season - last year British IndyCar driver Justin Wilson died after being hit by flying debris.
In the battle between the Mercedes drivers, world champion Hamilton was 0.722secs behind the German on the super-soft tyre and 0.21secs adrift on the soft.
That suggested Hamilton must have made a mistake on his brief run on the super-soft tyre, which will be used in the battle for pole position on Saturday, although it was not immediately obvious where or what.
Vettel, the faster of the two Ferraris, was 1.048secs behind overall, but the Italian cars only ran the soft tyre, on which the German was just 0.078secs slower than Rosberg.
However, Vettel, who had a couple of spins early in the session, set his time later on when the track would have had more grip and would therefore have been faster.
The four-time champion was among several drivers to fall foul of the low-grip surface on this rarely used track around the 2014 Winter Olympic stadiums in Sochi on the Black Sea coast.
Among the others was McLaren's Jenson Button, who also had two spins, at the same places as Vettel.
The Englishman ended up 12th fastest, while team-mate Fernando Alonso was 0.108secs behind in 13th.
The Spaniard lost a large chunk of the final third of the session when he was called into the pits with a technical problem, but managed to get out for a final run at the end.
Second Russian Grand Prix practice begins at 12:00 BST, with coverage online and 5 live Sports Extra.