George Russell was left confused by Mercedes' lack of pace during the sprint race in Brazil and admitted it was “tough to swallow”.
Russell started the race in fourth place and moved up to second by the end of the first lap.
However, he was soon shuffled back behind Lando Norris and Sergio Perez and crossed the line 26 seconds down on race-winner Max Verstappen.
Russell's team-mate Lewis Hamilton also endured a challenging race as he struggled with his tyres and slipped from fifth to seventh.
Mercedes' lack of pace comes after it showed strong form in the United States and Mexico, leaving Russell confused over its performance in Brazil.
“It’s tough to swallow and really quite confusing,” Russell told Sky Sports.
“It's always down to these tyres and getting in the sweet spot. I think, especially these last two weekends, nobody has really understood when it's working for them.
“We saw with Ferrari in qualifying last week and then we found our pace in a race.
“Suddenly it wasn’t there for us [in the sprint race] when we expected quite a lot. So I don't know going into [the Grand Prix].
“Right now obviously, we're not too optimistic. Even Max, we've never been that far behind.
“He finished 25 seconds ahead in a 24-lap race. So over a second a lap, that was really quite surprising for us.”
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Sunday's Grand Prix is set to be held in cooler conditions with Hamilton and Russell starting from fifth and sixth respectively.
Hamilton expressed belief that he doesn't think the cooler track surface will benefit Mercedes, but Russell is hopeful that it will help the Brackley-based squad.
“I'm 100 per cent sure it will look different tomorrow because it's three degrees different which sounds like nothing and should be nothing.
“But in these F1 cars with these tyres, it's everything. So fingers crossed it changes everything for for the better for us. Of course there are no guarantees.”