Nico Rosberg says being forced to revert to his former Mercedes power unit cost him a shot at challenging Lewis Hamilton for pole position in the Italian Grand Prix as he anticipates an equally tough race against his title rival and the Ferraris.

With Hamilton and Rosberg using Mercedes' new upgraded power unit this weekend, issues with Rosberg's during FP3 forced the team to revert to his Spa-specification engine ahead of qualifying.

With a gulf in mileage between the two units, Rosberg couldn't match the pace of his team-mate - falling three tenths short in Q3 - and would also lose out to Kimi Raikkonen and Sebastian Vettel, themselves using an upgraded Ferrari power unit this weekend.

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With two cars between them and a 28 point deficit against him too, Rosberg now faces a task coming into race day, not least because he expects the engine to prove a 'very big compromise' over a longer distance too.

"We had to go back to an engine that has done six races now and, every kilometre, you lose a bit of power so," he told Sky Sports. Especially at a track like Monza, the absolute power track where you need a good engine, so it's very disappointing to have it happen here.

"It's a very big compromise and the reason why I am fourth today, which makes the race tomorrow tough as I will be slower in the race tomorrow as well. That's not ideal and will be difficult against those Ferraris."

Reflecting on the form of Ferrari, Rosberg says he isn't necessarily surprised, but insists he would have been ahead but for the engine.

"I had the old engine so I had the power that's available on that engine - I'm not sure what Lewis did or whether they had to turn him down or not."