Formula One: Rosberg on top as engine problems cost Hamilton an hour

MONZA, Italy - Championship leader Nico Rosberg topped the times ahead of title rival and Mercedes team-mate Lewis Hamilton in Friday's second free practice session for the Italian Grand Prix.

The German, who leads Hamilton by 29 points in the title race, clocked a best time of 1min 26.225sec early in the 90-minute session, long before Hamilton, hampered by electrical problems, entered the fray with only half-an-hour remaining.

Hamilton, keen to make up for lost time spent waiting for repairs to sensors that had prevented his team from starting his engine, was swiftly on the pace and finally clocked a lap that was only 0.061sec adrift of Rosberg.

This was clear evidence of his pace on a day when he had shown masterful speed in the morning to top the opening practice times.

He was fastest in both of the first two lap sectors, but was held up by traffic in the third - enough to show he had the raw speed to be fastest if he had a trouble-free run.

Finn Kimi Raikkonen was one-tenth of a second down in third place in the leading Ferrari ahead of his team-mate Fernando Alonso as the 'scarlet scuderia' enjoyed their traditional Friday form at the old Autodromo.

Fast-improving Finn Valtteri Bottas was fifth for Williams, ahead of Britain's Jenson Button in his McLaren, with defending four-time champion Sebastian Vettel of Germany showing improved form for Red Bull back at the circuit where he gained his maiden victory in 2008.

Denmark's Kevin Magnussen was eighth in the second McLaren ahead of Brazilian Felipe Massa in the second Williams, with Australian Daniel Ricciardo, in the second Red Bull, taking 10th place in a session of improving times on a warm, if overcast afternoon.

Mexican Sergio Perez had set the early fastest time in his Force India car, powered by Mercedes, before Rosberg took over at the top as the Mercedes factory team, at times, stood to form a human wall at the front of their garage, while mechanics worked furiously to discover what it was that was stopping Hamilton's car from firing up.

The 2008 champion has not had a clear fault-free weekend at any Grand Prix since the Spanish Grand Prix in May, and in that run he has been hampered by a series of problems including brake failures and fires.

But it was his collision with team-mate Rosberg, on the second lap of the Belgian race two weekends ago, that remained the top topic for gossip in the paddock as Mercedes team chief Toto Wolff warned his drivers they were in danger of losing their jobs if they continued to behave inappropriately.