By Alan Baldwin
SOCHI, Russia (Reuters) - Formula One championship leader Lewis Hamilton led the way as Mercedes ended the first day of Russian Grand Prix practice with a dominant one-two on the timesheets while Ferrari struggled.
The Briton, 40 points clear of Ferrari's Sebastian Vettel with six races remaining, lapped Sochi's 5.8 km Olympic Park circuit with a best time of one minute 33.385 seconds on the hypersoft tyres.
Vettel ended up only fifth-fastest, in 1:33.928, and behind the two Red Bulls driven by Max Verstappen and Daniel Ricciardo. His Finnish team mate Kimi Raikkonen was sixth.
The German had been fastest in the morning with a best time of 1:34.488, also on hypersoft tyres, with Hamilton third on the slower softs.
Vettel is running out of time to rein in Hamilton, who has seven victories to his name this season, including four of the last five.
Mercedes have won every Russian Grand Prix since the first in 2014, with Hamilton twice victorious and team mate Valtteri Bottas winning last year after now-retired Nico Rosberg in 2016.
Verstappen turns 21 on Sunday but cannot become the first 20-year-old to start on pole position due to a raft of engine penalties.
The Dutch driver is one of five listed with confirmed drops to the back of the grid, leaving the exact starting order uncertain.
Verstappen's Australian team mate Daniel Ricciardo, Toro Rosso's Pierre Gasly and Brendon Hartley as well as McLaren's Fernando Alonso were the others hit with grid penalties.
Vettel took pole in Sochi last year but Bottas made a lightning start to set up the win. The Finn was 0.199 slower than Hamilton in Friday's dry afternoon session.
Ricciardo, fourth after lunch, had sat out much of the morning with mechanics working on his car.
Italian reserve Antonio Giovinazzi, who will step up to the Sauber race seat next season, took Marcus Ericsson's car for the opening practice while British teenager Lando Norris gained experience ahead of his McLaren debut next year.
Giovinazzi was 10th fastest, with 18-year-old Norris -- who went off but avoided the barriers -- 13th.
McLaren regular Stoffel Vandoorne, who is ceding his place to the Briton at the end of the year, was only 16th.
Renault gave Russian F2 driver Artem Markelov (15th) a run in Carlos Sainz's car while Canadian Nicholas Latifi (17th) was in Sergio Perez's Force India.
(Reporting by Alan Baldwin; editing by Christian Radnedge and Kevin Liffey)