Russian GP: Fuel worries caused Toro Rosso slump in the race

Home favourite Kvyat started fifth for Sochi's first F1 race, with team-mate Vergne ninth on the grid.

But they tumbled backwards in the grand prix, finishing outside the points in 13th and 14th, with Vergne ahead.

"After the first stop, I couldn't find the grip on the tyres, everyone went and the positions were more or less stabilised," Kvyat told NBC Sports.

"But then we went onto fuel management, and it was a little bit too much compared to the other teams, probably with Mercedes engines, and that put us a little bit behind.

"I had a flat spot which caused an extra stop, and it was very, very difficult.

"I don't think points were possible.

"In the first stint I got caught out with people in front of me, and it all started to be bad from the beginning."

Vergne added: "We had to care too much about our fuel consumption compared to the others. This is something that we will have to understand."

Team boss Franz Tost said it was hard to understand why fuel had been such a big problem for Toro Rosso.

"After around 20 laps, we had to start saving fuel, so we could no longer use our real potential today," he said.

"We lost places just because several other cars could easily overtake us, therefore the final result is actually quite disappointing.

"Now we have to analyse why it was possible that, with the same amount of fuel for everybody, we were so far behind.

"We need to find a solution and prevent this from happening again."