After elation at victory in the last round in Singapore that delivered a commanding 28-point lead in the world championship, Lewis Hamilton was adamant he was taking nothing for granted before the final six races. On early form in Malaysia, he was absolutely right to do so.

The season has ebbed and flowed between the British driver and his rival Sebastian Vettel, and in Sepang on Friday it was Hamilton who struggled with a car suffering from a “fundamental” problem. Mercedes have little time to turn it round before a difficult day in practice becomes a potentially disastrous weekend.

Banishing thoughts of past failures is second nature to those at the top of their sport but Hamilton will have to work hard to ensure he does not allow any sense of foreboding through his natural defence mechanism. Last year an engine failure while leading this race was a crushing blow to his championship ambitions and Nico Rosberg took the title.

This year he came to Sepang on top, with a strong points lead at a circuit that should suit his Mercedes strongly over Vettel’s Ferrari. The long straights and high-speed corners echo Silverstone and Monza, where his car was dominant. When he took it out, however, Hamilton discovered not the car of the British Grand Prix but rather that of Russia and Monaco – where the setup, balance and grip had eluded him.

The first practice session was run in the wet but come dry conditions in the afternoon Mercedes were presented with a mystery. Vettel was quickest, with a new lap record, and followed by his Ferrari team-mate Kimi Raikkonen. Hamilton could manage only a distant sixth, a full second and a half back and was clearly struggling for grip – losing the rear and heading across the gravel though turn seven.

The timing sheets had, understandably, set alarm bells ringing. “It’s been a very difficult day,” said Hamilton. “I’ve been struggling with the car, 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.”

The team have previously described their car as something of a “diva”. It had proved difficult to master the reasons why it was working so well at certain tracks and so poorly at others. After Monaco they were confident they had largely solved the conundrum but Sepang has proved once again there is work to be done. The issue does not appear to be straightforward and is being taken wholly seriously by the team.

“The car seems to be unbalanced and that triggers an awful lot of consequences, it was certainly one of the worst Fridays I can remember,” said the Mercedes executive director, Toto Wolff. “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.”

The team had brought new parts to Malaysia and must examine whether they have contributed to the problem, which could not simply be narrowed to a specific set of tyres, temperature or fuel load. They were off the pace in the wet and dry and on both short and long runs, as Wolff admitted. “The lap time is reality, the stopwatch always tells the truth,” he said. “The lap time today showed that we are not quick enough.”

A stable car that they can push is the goal, something Vettel already has beneath him. He had led the championship at every round until Monza but the first corner crash in Singapore, eliminating him from the race, proved hugely costly. Closing the gap to Hamilton is paramount. He knows it and has psychologically reset since the last race.

“The way Singapore happened you can look at it again and again; it doesn’t matter,” he said. His demeanour mirrored that of Hamilton after this race last year – a sense of acceptance of the position and a determination to do everything within his power to continue the fight.

“We have to go here, and that’s the good thing about racing, the part that I enjoy,” he said. “That’s what you fight for, you fight for every centimetre, every tenth that you can find somewhere, over a lot of laps in the race or over one lap in qualifying, to make the difference. I’m sure that, as a team, we can make that difference.”

Thus far Ferrari have done just that – their revised bargeboard and floor package was working well. Their pace was impressive regardless of everybody else’s performance. Should Mercedes be unable to find what is making the difference in their garage by qualifying on Saturday morning, Vettel could be in the best possible position to ensure Hamilton endures another disappointing weekend in Malaysia.