Championship-chasing German Sebastian Vettel topped the times in Friday's second free practice session ahead of this weekend's potentially-decisive Singapore Grand Prix.

The 24-year-old Red Bull driver clocked the fastest lap in one minute and 46.374, to finish the day two-tenths of a second ahead of nearest rival and two-time champion Fernando Alonso of Ferrari.

Briton Lewis Hamilton of McLaren, quickest in an incident-packed opening session, came third in the later run ahead of Brazilian Felipe Massa in the second Ferrari. The first practice was delayed because of problems with kerbing along the Singapore street-circuit and when proceedings finally began 30 minutes late, it was punctuated by red-flagged interruptions for various minor incidents.

The second period was more straightforward as the teams acclimatised to the choking heat of 32 degrees Celsius and heavy humidity, with the main talking point being Jenson Button's mid-practice withdrawal. His participation in the session ended after he almost lost control of his McLaren under braking at Turn 14.

Adrian Sutil of Force India was seventh ahead of Mexican Sergio Perez.

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