Both Lewis Hamilton and Valtteri Bottas say that their Mercedes Formula 1 team "have work to do overnight to fine-tune the balance" of its W08 Hybrid car after Friday practice for the Hungarian Grand Prix.
Neither driver topped a session, with Red Bull's Daniel Ricciardo topping both First and Second Practice, with Hamilton recording third and fifth in the two sessions, with Bottas managing fifth and third respectively.
"The three teams at the front seemed very close," said Bottas, "we have some work to do."
"I was not quite happy with the balance of the car today and I was especially struggling with the rear end.
"The temperatures are expected to rise tomorrow, which means that we really need to get the rear of the car more stable.
"I think that will be the main area for us to focus on."
Rear-stability is key at the Hungaroring owing to the heavy traction zones, in particular, those at Turn 1 and Turn 14.
A car that has a stable rear can put the power down earlier, and also have better rear tyre life, which could be crucial this weekend, as the margin between a one-stop race or a two-stopper looks fine.
"There's clearly good pace in the car"
Hamilton, who failed to get a super-soft tyre time in during Second Practice, owing to him aborting the first attempt and then Pascal Wehrlein bringing out the red-flag said that: "It wasn't the easiest start to the weekend, with conditions very gusty.
"We end the day in fifth, but there's clearly good pace in the car.
"There's some more work to do overnight to fine-tune the balance to the get the car just where we want it and I believe the pace is in there.
"We just need to unlock it ahead of qualifying because every tenth is going to be crucial with three teams in the mix."
Set-up issues are back
The car badly struggled to get temperature in its tyres during the Russian Grand Prix weekend, and Bottas was better able to cope with this than Hamilton to take his first win, and again in Monaco, when Hamilton slipped out in Q2 and could only muster seventh in the race.
After Monaco, where constructor championship rivals Ferrari finished 1-2, the Italian marque was leading the standings from the German team.
However, Ferrari have not won since Monaco, whereas Mercedes have won three of the four races, only losing in Azerbaijan because of the headrest issue for Hamilton.