Hamilton laments his struggles as rival Vettel fires out an early warning
Lewis Hamilton bemoaned a "very difficult" start to his Malaysian Grand Prix weekend as title rival Sebastian Vettel set the pace at Sepang.
Vettel clocked a new lap record to go quickest in second practice with his Ferrari team-mate Kimi Raikkonen second.
Meanwhile, Mercedes endured what their head of motorsport Toto Wolff labelled "one of the worst Fridays I can remember" as Hamilton and team-mate Valtteri Bottas struggled.
Both span in the afternoon session as they were restricted in their running time due to a number of outside factors.
First practice was delayed due to a heavy downpour and there was no running for the first 30 minutes.
The second session was curtailed by more than 20 minutes as a red flag ended running after the Haas of Romain Grosjean had a big accident as it suffered a major puncture from hitting an uncovered drain.
Vettel is now 28 points behind Hamilton in the fight for the title after the Briton's hat-trick of wins, but the German shone here.
He was almost 1.5 seconds quicker than Hamilton, who was down in sixth, leaving the championship leader hopeful of improvements today.
"It's been a very difficult day," said Hamilton. "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.
"We're hoping that we're able to find our bearings overnight and regroup for tomorrow."
Wolff, meanwhile, insisted Mercedes' struggles were real and they weren't saving their true pace for today.
"The lap time is the reality," he said. "The stopwatch always tells the truth. The lap time showed we are not quick enough.
"The car seems to be unbalanced and that triggers consequences. It is one of the worst Fridays I can remember."
Grosjean crashed out when his Haas hit a drain on turn 13 which had seemingly had its cover ripped off moments earlier as Hamilton passed over.
The Frenchman suffered a massive puncture as his rear-right tyre bumped over the uncovered drain and saw him hit the wall at high speed.
He was able to walk away from the car and confirm he had not been seriously hurt.
Race director Charlie Whiting revealed he and his team faced a long day to get the circuit into a safe state ahead of free practice three this afternoon.
"Unfortunately one drain grate has come up. It is an inspection hatch, a drain that runs all along the curb," he explained.
"One has come up, they are normally bolted and welded but it seems it has just come away.
"For the rest of the day we will concentrate on getting the four kerbs welded back down again...they have probably become a bit fatigued. We have to get it fixed for tomorrow of course."