Lewis Hamilton is hoping for rain in Sochi on Saturday, or else the outlook for another Mercedes win at the Russian Grand Prix could look a little gloomy.

Reigning world champion Hamilton has won three of the five editions of the race, including 12 months ago, since it was added to the Formula One calendar in 2014.

Nico Rosberg (2016) and Valtteri Bottas (2017) have also triumphed for the Mercedes at the track, though their run of dominance appears under threat this weekend, most notably from Ferrari.

Charles Leclerc - who triumphed in Belgium and Italy before team orders forced him to settle for second, behind Sebastian Vettel, in Singapore last weekend - impressed in the two practice sessions on Friday, leaving Hamilton hoping for some help from the weather.

The British driver finished fifth in FP1 and while he improved second time around, moving up a place in the standings, he expects to be up late working on the car in an attempt to close the gap.

"It's going to be tough day tomorrow, but I hope it's going to be raining. I heard it's going to be raining!" Hamilton said, according to quotes on the official F1 website.

"The pace was a bit better in P2, but still not where we want it to be but we'll work hard tonight.

"I'm always at the track till super late, we'll work late to understand and then go away and come back and make some changes to the car."

Hamilton admitted the pace of the Ferrari cars is a cause for concern, even though there were some promising signs for Mercedes.

"It's been a day of discovery and exploring," he said. "It started off not too bad and then it didn't really improve as with the others, [who] made some big progressions.

"[As] we anticipated, we’re losing eight tenths of a second to the Ferraris on the straights.

"Nonetheless, we have been improving, but they are improving at a serious rate. We're just trying to figure out how we can improve the car, but it’s not an easy task."