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The Williams Formula 1 team admits it was caught out by Mercedes' tyre life in Valtteri Bottas and Nico Rosberg's Russian Grand Prix battle.

Rosberg managed to complete all but one of the Sochi F1 race's 53 laps on a single set of medium Pirellis without losing pace following his lap-one error, recovering to second.

When Rosberg overtook Bottas just after half-distance, Williams reassured its driver that he would be able to retaliate later in the race.

Williams performance chief Rob Smedley admitted no one expected Rosberg to last the distance on that strategy.

"We were ultimately very surprised by that," Smedley conceded.

"We thought there was a chance we would be able to get to the end of the race on a set of tyres, but you wouldn't be very quick at the end.

"Valtteri was throwing in some fast laps, but Nico was pretty much matching him on 25-lap-older tyres.

"In qualifying, if Valtteri had put together a perfect lap, he was going to be second and very close to pole position.

"In the race, they just had much longer legs than us - they could just basically stroke it out and manage their tyres."

MASSA BAFFLED BY PEREZ'S PACE

Felipe Massa finished only 11th in the other Williams, despite following Rosberg through the field early on after fuel pressure issues left him starting 18th.

Massa said getting stuck behind 10th-placed Sergio Perez's Force India ruined his day.

"The pace was very good - me and Rosberg were overtaking so many cars so easily, but when I got to Perez it was not possible to pass," Massa explained.

"I got DRS on the straight, but I couldn't catch his car enough to try, and that destroyed my race.

"It was so easy to pass everybody, even the Toro Rossos, but not him - he was the only car."

The Brazilian also questioned Williams's decision to pit him at the end of the opening lap, but Smedley said he had expected the performance gap between the tyre types to be bigger.

"Pretty much all weekend we'd seen one second between the tyres and negative degradation on the option [soft]," he said.

"We wanted to stop him and get him off that [medium] tyre as soon as possible and onto the option.

"I think if we hadn't got stuck behind Perez then we would have been able to get on the back of the Ferraris."