An increasingly frustrated Nico Rosberg has called upon Mercedes to investigate the spate of reliability issues that have crept into the championship-winning team in recent races after they played a part in all-but-eliminating him from this year's title contention.

The German was forced to retire from the lead of the Russian Grand Prix on lap five when his throttle pedal shifted and began to stick through the corners, the exit marking his second DNF - and a third retirement for a Mercedes driver - in four races.

The two exits have all-but-dropped Rosberg out of the title reckoning, with a 73 point gap to team-mate Lewis Hamilton - who suffered his own technical-related DNF in Singapore - and only a maximum of 100 points remaining from the final four races.

The failures go their way to sully an otherwise dominant year for the Mercedes AMG team, which celebrated winning the constructors' title in Russia, with a frustrated Rosberg saying he is 'surprised' the team is experiencing 'worse' reliability at this stage in the season.

"It's a strange one," he said. "Just when we were looking so good on reliability to then have a few issues. It is surprising and is has gotten worse through the season. The last couple of months we have had more issues and race stoppers, so we must look into it again and continue to improve in that area because it seems we haven't made as big steps as we thought."

Indeed, Rosberg - who slipped behind Ferrari's Sebastian Vettel in the standings as a result of his Sochi retirement - feels aggrieved by the 'bad luck'.

"It's disappointing to see how this year has gone... a lot of bad luck in the last couple of months just when I needed to launch an attack, one thing after another. Small things also, major things, race stoppers just derailed it in the last months and that has been tough."