Nico Rosberg says a direct pace and performance comparison between Mercedes and Ferrari's updated power unit isn't realistic as his team has its engines turned down until qualifying.

The Mercedes pair of Rosberg and Lewis Hamilton topped a practice session each on the opening day of track action at Monza, with both Ferraris drivers acting as their nearest rivals with around four-tenths separating the two teams.

ITALIAN GRAND PRIX - Free practice results (2)

Rosberg, who was delighted by his progress and set-up work, is intrigued to understand the true pace of both manufacturers at the Italian circuit but feels any comparisons made from Friday's free practice isn't a fair reflection after admitting Mercedes were running with a toned down engine.

"Ferrari seemed to have made progress with their engine but we had our engines turned down today, so we cannot compare directly," Rosberg said. "Let's wait for qualifying to see where we all are. I'm optimistic for qualifying, confident I'll be ahead but they're not too far off.

"Today we started straightaway from the right place, so it felt good out there. It's been a good battle with Lewis today, one time he was quicker, one time I was quicker, and behind us Ferrari seems to be the closest team to us.

"The tyres seem to be working much better than expected, whereas in Spa they were overheating too much. This was a good day for us, I felt really good in the car straight away, we got the balance pretty much right. We ran with all three compounds available this weekend - medium, soft and super soft - and they're all a possibility for the race."

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Rosberg will be aiming to retain his momentum from Spa, where he claimed his first win since Baku, as he looks to continue to eat into Hamilton's nine-point lead in the F1 drivers' championship.

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