Toto Wolff feels Mercedes' dominance of the F1 2016 championship has been constantly come under pressure from the rejuvenated Red Bull while he isn't ruling out the threat from Ferrari.

Mercedes looks set to better its imperious record from last year when it failed to win just three races all season, with Lewis Hamilton and Nico Rosberg taking 11 of the 12 victories available before the summer break.

The only race where a Mercedes failed to find the top step of the rostrum in 2016 was at Spain where Hamilton and Rosberg collided with one another on the opening lap, while Hamilton improved on the team's 2015 record by grabbing victory in Hungary where 12 months ago the team was trumped by both Ferrari and Red Bull.

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Despite the dominance, Wolff feels the results have been far from assured or predicted and where Mercedes has perceived strengths the team principal believes its rivals are closing the gap.

"We were expecting Hungary to be a difficult one and Red Bull to be performing well and it was the opposite. We were actually very strong as well as in Silverstone and Hockenheim should have been won easier for us and it happened not to be," Wolff said. "So I think it isn't clear cut.

"Nevertheless they keep coming back. There wasn't a lot of speed differential on the straight anymore so they will be coming back in full form next year and even for the second half of the season I have no doubt."

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"This is what I keep saying, it's a pretty unusual situation, the team is winning nearly every race. We have kept it until now, nearly two and a half years and it's not going to last for ever."

Wolff has also warned of the potential danger Ferrari still holds despite slipping behind Red Bull in the F1 constructors' championship in Germany.

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The Italian manufacturer was the only team to beat Mercedes last year as Sebastian Vettel claimed three race wins at Malaysia, Hungary and Singapore respectively.

"I don't think you can write Ferrari off, they have clearly had a tough time most recently," he said. "I think they switched off very early for 2017. Never underestimate a team with so much resources and so much passion."