SOCHI, Russia (Reuters) - The Formula One constructors' championship is won for the second year in a row, and Lewis Hamilton is on the brink of his third world title, but lack of reliability is still giving Mercedes plenty to worry about.

The champions have suffered three retirements in four races and the throttle failure that sidelined Nico Rosberg in Russia on Sunday took the gloss off another triumphant afternoon for the team.

With Ferrari's Sebastian Vettel leapfrogging Rosberg into second place overall with four races remaining, 66 points behind Hamilton, Mercedes still have a battle on their hands to end the season with their drivers one-two in the championship.

Hamilton can wrap up his second successive crown in Austin, Texas, in two weeks' time -- perfect timing in a major market for the car company -- but reliability could still throw a spanner in the works.

"There is generally speaking no good luck and bad luck from my point of view," the team's non-executive chairman Niki Lauda told Reuters after race winner Hamilton had been congratulated by Russian President Vladimir Putin at Sochi's Olympic Park circuit.

"Nevertheless, our reliability performance in the second half of the season went down. So we have to try to fix that and analyze why and what.

"This throttle issue is a brand new worked perfectly for a long time so this is a surprise to everybody. But it will be fixed and will not happen again," added the Austrian.

Rosberg, now 73 points behind Hamilton, also retired in Italy with an engine failure while his team mate failed to score in Singapore due to a loss of boost pressure.

While Mercedes team bosses and drivers were due to thank the assembled staff at the factory at Brackley in England, and the engine side at nearby Brixworth, they will want to waste no time in addressing the other issues.

"It is a strange one. We were looking so good on reliability then to have quite a few issues. It is surprising and it has gone worse and worse through the season especially in the last few months," said Rosberg.

"We must look into that again and continue to improve that area because it seems we have not made as big steps as we thought."

Mercedes have taken 12 wins in 15 races, with eight one-twos, but Ferrari have also closed the performance gap.

"Reliability, of course, is something we've focused on a huge amount over the last year and a half," said Hamilton after his ninth win of the season.

"For whatever reason...we are struggling or having a little bit more problems with it and Nico's been incredibly unfortunate to suffer the worst.

"I know we’ll be working as hard as we can to rectify it and hope that in these next races we don’t have a problem, but of course it is a concern for us."

(Reporting by Alan Baldwin, editing by Amlan Chakraborty)