Nico Rosberg will line up fourth on tomorrow's Italian Grand Prix grid, behind Lewis Hamilton and the two fast-starting Ferraris of Kimi Raikkonen and Sebastian Vettel.
The German driver's chances of challenging his Mercedes team mate for pole position were reduced from the outset when his new upgraded power unit showed signs of weakness in this morning's free practice.
As a measure of precaution, Mercedes reverted back to Rosberg's older-specification Spa engine for qualifying.
"We had to go back to an engine that's done six races, and with every kilometer you lose a little bit of power, Rosberg said.
"At Monza, which is the absolute power track, you need a good engine. So it's very disappointing to have this happen here as it's a really big compromise. It makes the race tomorrow tough as I'll be a bit slower than I should. It's not ideal and it will be difficult against the Ferraris."
Rosberg also complained of handling issues which affected his performance in Q3, believing therefore that Ferrari's good performance was the result of his own Mercedes' relative weakness rather than the Italian squad's form at its home race.
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