SINGAPORE: Sebastian Vettel led final free practice for the Singapore Grand Prix on Saturday as Lewis Hamilton's bid for a record-equalling eighth straight pole position looked in serious doubt.

Ferrari's Vettel clocked 1min 45.682sec late in the dusk session, more than a second quicker than championship leader Hamilton of Mercedes, who wound up fifth fastest.

Hamilton is trying to equal his idol Ayrton Senna's record of eight poles in a row, while dominant Mercedes have been the fastest qualifiers in all 12 grands prix so far this year.

However, their struggles continued at the tough, technical Singapore street circuit with Nico Rosberg coming in one place below Hamilton in sixth.

Vettel's team-mate Kimi Raikkonen timed second quickest to complete a Ferrari one-two, followed by the Red Bulls of Daniil Kvyat and Daniel Ricciardo.

McLaren's Fernando Alonso was seventh fastest but his sparring partner Jenson Button, who is in the midst of contract negotiations, was down in 13th.

American debutant Alexander Rossi again got the better of his team-mate Will Stevens, lapping 2.7 seconds quicker than his bottom-placed fellow Manor driver.

Hamilton's bid for a 12th pole position in 13 qualifying sessions this year begins at 9:00 pm (1300 GMT) under floodlights at the downtown layout. th/jom