Mercedes Formula 1 boss Toto Wolff says his team's "very difficult" Friday practice sessions for the Malaysian Grand Prix is down to a "fundamental issue" it needs to solve.
Lewis Hamilton and Valtteri Bottas struggled in both the wet-weather FP1 and the dry FP2, placing no higher than sixth in either session.
As Ferrari's Sebastian Vettel swept to the fastest time in FP2, both Mercedes drivers were 1.4s off the pace, with Hamilton sixth and Bottas seventh.
Speaking to Sky Sports F1 after the second session, Wolff called it a "very difficult session and said the team "struggled from the get-go - in the wet, in the dry, on all tyres".
He added: "We have to find out overnight what it is.
"At the moment the car is unbalanced and you slide all over the place, the fronts and the rears, you overheat them and then nothing goes any more.
"There is a gremlin in the car.
"You can compare Malaysia a little bit to Silverstone, where we have been very good.
"We've been very good in the high-speed [corners], we are not at all good in the high-speed here.
"There is a fundamental issue in the car that we need to find out.
"They are very complex, so it can be a tiny detail that just unsettles the car."
Mercedes' 2017 challenger, the W08, was dubbed a 'diva' earlier this season due to a tendency to vary between allowing the team to dominate and forcing it to accept second-best behind Ferrari.
Echoing the sentiments of his team boss, championship leader Hamilton described his Friday at Sepang as "a very difficult day".
"I've been struggling with the car today, so we have to review and try to understand where we have gone wrong with the balance," said Hamilton, who leads Vettel by 28 points.
"We're hoping that we're able to find our bearings overnight and regroup for tomorrow."
Bottas ended both sessions right behind his team-mate in the order.
"We're definitely lacking some performance, it seems like Ferrari and Red Bull are very strong," said the Finn.
"So we have some work to do if we want to be on the front row tomorrow.
"The car balance itself didn't feel too bad; it's just a question of overall grip.
"The key area we are going to need to focus on tonight will be getting the maximum out of the tyres over one lap."