By Alan Baldwin
SOCHI, Russia (Reuters) - Formula One champions Mercedes were facing the prospect of a third defeat in four races after Ferrari ended two of the team's longest sequences of success at the Russian Grand Prix on Saturday.
For the first time in 31 races, since Singapore in September 2015, grand prix fans will see a starting grid without a Mercedes on the front row.
Germany's four-times world champion Sebastian Vettel, on pole, and Finnish team mate Kimi Raikkonen gave Ferrari their first front-row lockout since 2008 and ended Mercedes's run of 18 successive pole positions.
On Sunday, the Mercedes boast that they have led every lap of every race in Sochi since the first in 2014 also looks destined to disappear.
Mercedes boss Toto Wolff, whose team have won both championships for the past three years, said it had been bound to happen eventually.
"We had so many pole positions, so many front-row lockouts, 52 race victories (in 62 grands prix) in three years and we knew at a certain stage it's going to change," the Austrian told reporters.
"Now this is exactly the challenge that we embrace. Every series ends. You cannot win forever, as much as we would love that."
Valtteri Bottas, the Finn who joined Mercedes in January as replacement for the now-retired 2016 world champion Nico Rosberg, qualified third with triple world champion Lewis Hamilton fourth.
Hamilton, who started the first two races of the season on pole and is seven points behind Vettel, looked strangely error-prone as he struggled with an unusually skittish car.
With Mercedes enjoying one-lap supremacy in qualifying up to now, but Ferrari impressive over the race distance, the champions are up against it.
"We have seen that they have been very strong on race pace in these last races," said Wolff.
"We will do whatever is in our hands to win the race but it's going to be very difficult... Ferrari have done a very good job over the winter and it's just the two teams that are miles ahead of everybody else," he added.
"Now we just need to be rigorous in the analysis of what's missing, put the dots together and out-develop Ferrari throughout the season. And this is not easy because they are a fierce competitor."
(Editing by Clare Fallon)