George Russell was surprised by the "unexpected cars" that were quicker than Mercedes during practice for the Mexico City Grand Prix.
The Mercedes driver was 10th quickest in the second practice session beaten by the usual pace-setters of Red Bull with both Ferraris and McLarens also ahead along with seventh-placed team-mate Lewis Hamilton.
However, both Valtteri Bottas and Daniel Ricciardo who finished the second session in fourth and sixth, respectively, showed impressive flashes of pace that took Russell aback, with the one-time Grand Prix winner only having half the running after stepping out of the W14 to allow Mercedes to fulfil one of its mandatory rookie sessions with Frederik Vesti in the first session.
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Russell: There is potential in Mercedes' package
"It was a bit of a tricky session with rain around here and there, we also had the prototype Pirelli tyre and it's a very tight field out there," Russell explained to F1 TV.
"There is potential, I think we have a lot of work to do overnight as it's very tight, especially with some unexpected cars - we saw some cars in the top 10 that we weren't expecting to be there.
"Nobody would have expected [Alex] Albon to be super quick in FP1 (second behind Max Verstappen), Ricciardo and Bottas up there in FP2, so of course we want to be fighting for position and Max is always half a second ahead.
"We feel like we've got a lot to improve, but that there is the opportunity to do it as well.
"There are different challenges here in Mexico, it is all about getting the tyres in the right window, and then in the long run, you are managing the engine, the brakes and that has an impact on the tyres as well.
"There are many things that you are juggling out there and it is going to be a very tricky race and that is why qualifying is going to be key.
"If anybody can get it right, they can be fighting for the front two rows - and we need to make sure that is us."
Russell's best lap was a 1:19.227 which was over half a second slower than Verstappen, and a further two-tenths adrift of Hamilton.
"Lewis and I were running two different setups and we probably both weren't in the right window, so that's given us an indication that probably halfway between both is the right place to be," he added.
"It's always so frustrating in the moment to have a [bad] car and not have the best of days, but through those difficult moments you learn probably more than when the car is in a better place."