Lewis Hamilton revealed damage to his W13 prevented him from completing his long-run programme in Hungarian Grand Prix practice as Mercedes' "crazy swings" flummoxed the seven-time F1 champion in Budapest.
The Silver Arrows took a first double podium of the season last weekend at Paul Ricard but could finish only eighth and 11th in second practice at the Hungaroring, with George Russell ahead of Hamilton.
Ferrari's Charles Leclerc finished quickest, with Hamilton 1.1secs adrift at the end of the second hour of running, with the vagaries of the W13 prompting complaints over a lack of grip and balance.
"The car is a bit of a struggle," conceded Hamilton. "It is crazy how much it swings so much from track to track.
"Just trying to figure out how to get the car working at the moment, it is a little bit loose and not doing what we want it to do, so a difficult day.
"Nothing has changed on the car since last week and I am the same driver as I was last week but it is just, for some reason at this track, it is not working as well.
"But once we got it right, the gap is about the same as last week."
Hamilton reveals floor damage
As is customary during a dry practice session, teams swapped from low-fuel, soft-tyre qualifying simulations to high-fuel, medium- or hard-tyre runs in the final quarter of the hour.
But Hamilton was unable to complete his race simulation. Explaining why, he said: "I didn't get to run at the end because I sustained some damage on my floor so lost a lot of downforce and after that it was pretty tricky for the long-run pace.
"It is going to be a tough weekend but we will give it everything and see what we have got."