text: Rosberg has won the first three races of the season to build a commanding 36-point lead in the championship.
  Hamilton said: "Of course there is an urgency. I can't really afford to fall any further back than that.
  "Who knows how the weekend will go? It could be as bad as in the past but I'm hoping it will be positive."
  Hamilton has finished second, third and seventh in the races so far.
  Bad starts took him out of the reckoning for victory in the first two races in Australia and Bahrain and then in China technical trouble left him at the back of the grid, from where he was involved in a crash at the first corner.
  Formula 1 has introduced new rules this season that gives the drivers more control at starts, reducing the technical assistance they get from the car.
  And Hamilton said he had been encouraged before Sunday's race in Sochi by the fact he had made a good start in Shanghai from his position at the back of the grid.
  "I made the best start," he said. "At the end of the weekend the team analyse the starts and I had the best of everyone, they told me.
  "This is a circuit where you need that because I think it is one of the longest runs to Turn One and it is easy to get drafted so a good start would help."
  Rosberg, who has won won six grands prix in a row dating back to last October in Mexico, said he was expecting a fightback from Hamilton over the remaining 18 races.
  "I'm pleased with the way I'm driving," he said. "For me it's not six straight, it's three, because I only count from the start of this season.
  "I have been driving really well but my opposition had a really messy start and I definitely expect them to bounce back, especially Lewis. And it is going to be really tough, like always. There is a massively long way to go still."
  Rosberg said Ferrari would also be a tough challenge for Mercedes, despite their difficult start to the season.
  "I'm not sounding a note of caution, I'm just realistic," he added. "Ferrari among others haven't shown what they can do yet - as an example they took each other out at the first corner in the last race.
  "They are a lot quicker than they have shown and for sure they are very close to us."
  But Ferrari driver Sebastian Vettel said he believed the Italian team were still not quite on Mercedes' pace.
  "I hope we can be very close this weekend," the four-time champion said. "First of all have a smooth weekend and see where we are.
  "We know we are probably not close enough but the race is on Sunday and there is always a chance.
  "We expected Mercedes to be very strong. Of course it is important you score points at this stage of the season, but there is still a long way to go. Our feeling is good but we know we can ramp it up."
target: Lewis Hamilton says he must beat Mercedes team-mate Nico Rosberg at this weekend's Russian Grand Prix to kick-start his title defence.