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Mercedes has not made the step it had hoped for on reliability in Formula 1 this year, with its situation getting "worse and worse" according to Nico Rosberg.

At Monza that Rosberg was forced to revert to an older-spec engine following final practice after a leak was discovered in the cooling system that contaminated his upgraded power unit.

Three laps from the end of the race, Rosberg's old engine - that was already high on mileage - failed as the German was pushing for second.

In Singapore Lewis Hamilton's car suffered a freak failure when a metal clamp broke on the plenum that caused a boost leak.

Reliability a concern - Hamilton

After normal service was resumed in Japan with a one-two finish spearheaded by Hamilton, Rosberg was struck by another issue in Russia on Sunday when he was forced out of the race early on by a throttle pedal failure.

Even Hamilton encountered a problem with his rear wing late on in the race, although it was not enough of a problem to deny him the win.

Asked about the lack of reliability of late, a frustrated Rosberg said after the race: "It's a strange one.

"Just when this year it looked like we were looking so good on reliability, to then have quite a few issues, it's surprising and it's got worse and worse now through the season.

"In the last couple of months we've had loads more issues, race stoppers and things like that.

"We must look into that again and continue to improve in that area.

"It seems we haven't made as big steps as we thought."

The problems in Italy and Russia in particular mean Rosberg's title challenge is all but over as he trails Hamilton by 73 points with only 100 available.

"It's just disappointing to see how this year has gone," added Rosberg.

"There's been a lot of bad luck, just when I needed to launch an attack and for things to go the other way. It's just been one thing after another.

"Small things too, and some major things, have just derailed it in the last months, and that's tough."

Rosberg joined everyone from the Mercedes team at its factory in the UK on Monday to celebrate the team's constructors' championship success, which he said helped him recover from the disappointment of the weekend.

"It's been a tough couple of days for me - but being back at the factories and seeing all my colleagues so happy after the championship win really puts a smile back on my face," he said.