Sebastian Vettel top as Lewis Hamilton struggles in Singapore

Sebastian Vettel has won two races this season

Mercedes face a serious threat for pole position at the Singapore Grand Prix after Lewis Hamilton was only fifth fastest in final practice.

Ferrari's Sebastian Vettel was top from team-mate Kimi Raikkonen, Red Bull's Daniil Kvyat and Daniel Ricciardo.

Hamilton was a massive 1.120 seconds slower than Vettel and complaining about the handling of the Mercedes.

It is a stunning turnaround after Mercedes have dominated pretty much the entire season so far.

And the team insisted that they were not disguising their form.

"There is no hiding the potential here," Mercedes tweeted. "The pace of our rivals is genuine - and very quick."

Hamilton's team-mate Nico Rosberg was sixth fastest ahead of a stunning performance by McLaren's Fernando Alonso in seventh, just 0.014secs behind the German.

Alonso's team-mate Jenson Button was 13th fastest, nearly half a second adrift of the Spaniard.

Mercedes in trouble

Mercedes' lack of pace was one of the biggest surprises of the season - it is only two weeks since Hamilton romped to victory in Italy, with a clear pace advantage of about a second a lap over everyone else in the race.

The team have been defeated only twice all year, and both times in specific circumstances.

In Malaysia, Ferrari had better tyre wear and Vettel used it to do one fewer stop than Hamilton and beat him on strategy.

In Hungary, mistakes from Hamilton opened the door to Vettel and he took advantage.

With the caveat that this is only practice, the difference so far is that Mercedes seem simply to lack pace - although they may not have been running their cars at maximum performance.

"It will be a shock for Mercedes," said former Cosworth engine boss Mark Gallagher, acting as an analyst for BBC F1 this weekend.

The team seem to be struggling to get the best out of the tyres at Marina Bay - the single key aspect of being quick around the bumpy, twisty and demanding track.

Ferrari and Red Bull on form

Vettel was in stunning form - heading Raikkonen by 0.45secs, as the Finn was only 0.045secs ahead of the highly impressive Kvyat.

This is only the Russian's second race in Singapore and he admitted on Thursday that he had found the first one last year very difficult, the intensity of the challenge in the heat and humidity, around a long lap and a race of nearly two hours testing all the drivers.

But Kvyat has looked very good indeed all weekend, brushing the walls with verve and so far shading his highly rated team-mate Ricciardo.

Alonso on fire

There was another starring performance from Alonso, delivering on the promise McLaren gave that this would be their strongest race for some time.

The Spanish two-time champion is always strong at Singapore and to be only 1.5secs off the pace in a car that is still lacking significant power from its Honda engine - even if that deficit is less of a handicap in Singapore - was an outstanding effort.

Fernando Alonso finished seventh during the third practice session

Button, complaining of problems with the behaviour of his car, was a long way back.

But the midfield battle looks as tight as expected, with five drivers from the Toro Rosso, Williams, Sauber and Force India teams in the half-second gap between the McLarens.

Getting a clean lap will be key to deciding grid positions when qualifying begins under the lights of F1's only night race at 14:00 BST.

Practice results

Singapore Grand Prix coverage