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The impressive performance Mercedes' Formula 1 engine upgrade delivered during the Italian Grand Prix weekend is a "frightening" prospect for its rivals, according to Red Bull team principal Christian Horner.

Mercedes used all seven of its tokens to update its F1 power unit ahead of Monza and while Nico Rosberg was unable to use his after the team discovered a leak in the cooling system, Lewis Hamilton was untouchable all weekend.

The reigning world champion was 1.588 seconds quicker than Mercedes' closest challenger, Ferrari's Sebastian Vettel, in first practice, with the margin 0.759s in the second session.

F1's engine war comes alive at Monza

While Ferrari was closer in qualifying, Hamilton took pole and controlled the race, proving how much he was driving within himself when he increased his lead from 22.5s to 25 across the final six laps when asked to push.

"It looked quite frightening on Friday," said Horner of Mercedes' engine step. "Obviously they had some sort of issue with Rosberg.

"It looked like they wound the engines back a little.

"But certainly the step they made on Friday looked significant, quite worrying.

"They're in a league of their own at the moment.

"In the race, we saw the kind of pace Lewis had at the end there, on a scrubbed prime [tyre], and it was pretty awesome.

"So I guess he was just managing the gap throughout the grand prix.

"I think they are clearly up the road."

MOST 'PAINFUL' DAYS SHOULD BE OVER

Starting from the other end of the grid to Mercedes owing to Renault grid penalties, Horner says his drivers Daniel Ricciardo and Daniil Kvyat made the best of a bad situation at Monza.

Mercedes rejects Red Bull engine supply deal

When asked whether Ricciardo's eighth and Kvyat's 10th was the best Red Bull could've hoped for, given its power deficit, Horner said: "Yes, absolutely.

"Starting at the back, with the penalties we had on this track where you are totally exposed, we got the best out of the strategy, the pit stops were good, the drivers did a good job.

"Theoretically these [Spa and Monza] have been two painful circuits for us. Singapore should represent a better chance.

"At the end of the day horsepower on all the tracks is crucial, but it plays a bigger effect on a circuit like this than, for example, in Singapore."