Mercedes Formula 1 boss Toto Wolff says the team is "in the dark" after Lewis Hamilton struggled with an "all over the place" car in Styrian Grand Prix second practice.
Hamilton finished the session in sixth position, over 0.6 seconds slower than his team-mate and Austrian GP winner Valtteri Bottas, who was just edged out of top spot by Red Bull's Max Verstappen.
The session was approached differently by teams because rain is expected to wash out Saturday's action and, if qualifying can't then be held on Sunday morning, the grid will revert to FP2.
Wolff said the team hadn't gone for the full qualifying mode with its power unit, but turned the engine up more than it normally would on a Friday.
"We discussed it for a long time, but in the end we went into the session a bit half pregnant," Wolff told Austrian broadcaster ORF. "Not using the full power, but also not cruising very slowly.
"It wasn't full attack, but it wasn't half attack either - we already added a little bit more.
"Let's say, we drove on a performance level that we normally don't drive on Friday.
"We are a bit in the dark, especially with Lewis' time. There's a problem with the car.
"At the moment we don't know. The car is just all over the place in the slow and medium fast corners.
"But we have data channels for everything, and you have to take your time to look at that."
Asked what the team had up its sleeve for qualifying, Wolff said: "Not so much anymore. Compared to last week, Verstappen has improved, and especially the Racing Points have really improved.
"Everyone else is slower, just like us. But there is a lot of thinking and data to collect to see where the performance is."
Hamilton said he had no explanation for his struggles in the afternoon session.
"That I don't know, I can't really say anything," said the world champion.
"It felt relatively normal, but it was quite far off, so there's a lot of work that needs to go on in the background to try and figure out why.
"I was feeling good in practice one and the start of P2 felt pretty good, and then it just started to drop off.
"Others out there are quick, and Valtteri's obviously got good pace.
"Hopefully tomorrow it will be a bit better."
Regarding FP2 potentially forming the grid in the event of a qualifying washout, Hamilton said: "It will suck if we don't get to qualify, because it wasn't great, I'll be starting further back than even last week, so it will definitely make it challenging.
"I'm trying to do the dry dance, if there's a dry dance to make sure there's no frigging rain for Sunday."