Lewis Hamilton top in Russian GP practice as Ferrari struggle
Lewis Hamilton headed Valtteri Bottas in a Mercedes one-two in second practice at the Russian Grand Prix, as title rival Sebastian Vettel struggled.
Hamilton pipped Bottas by 0.199 seconds while Vettel was fifth fastest in the Ferrari, 0.543secs off the pace.
Vettel, who had a spin late in the session, starts the weekend 40 points behind Hamilton, with a maximum of 150 available over the remaining six races.
The Red Bulls of Max Verstappen and Daniel Ricciardo were third and fourth.
They will start from the back of the grid because Red Bull have fitted new Renault engines to both cars and, as they have exceeded the maximum number of engine parts, that means they will receive grid penalties.
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Bad news for Vettel
Mercedes' pace on a single lap suggested they have an advantage over Ferrari at the start of the weekend, which is bad news for Vettel, who really needs to beat Hamilton this weekend to revive his title hopes.
Vettel set the pace in the first practice session, but he was only 0.33secs quicker than Hamilton despite using the softest hyper-soft tyre while the Mercedes was on the ultra-soft.
The time difference between those two tyres is in the region of a second - so both sessions told a similar story, of a Mercedes that has a chunk of time over Ferrari.
However, Friday practice times can be misleading, as it is not clear what specification the cars are running in, so it would be premature to draw any conclusions.
With the race expected to feature just a single pit stop, and overtaking difficult, grid positions are important - although the long run from the start to the first corner gives drivers on the second row a chance to get a tow and pass into the first corner, as Bottas did on his way to victory last year.
Hamilton said: "Sochi has been one of the weaker circuits for me in the past, particularly last year. So I've done a lot of work to understand the balance and see where I can improve to try and rectify this - today has been good in that sense."
"We look quite far away from the rest of the field," said Vettel. "Struggling a bit on one lap but also on the long run we went through our tyres a bit harder and quicker than the rest.
"But just looking at ourselves it was not a good day in terms of feel for the car. There is more we need to get to. We have some catching up to do."
Both Toro Rosso drivers - who have new, upgraded Honda engines this weekend - and the McLaren of Fernando Alonso, also with a new Renault engine, will join the Red Bulls at the back of the grid.
And the auguries looked good for the new Honda power-unit.
Pierre Gasly's Toro Rosso was just pipped for the best of the rest slot in seventh by 0.015secs by Force India's Sergio Perez, whose team-mate Esteban Ocon was 0.01secs behind his fellow Frenchman.