Speaking ahead of the governing FIA's awards ceremony in Russia, the Austrian told reporters he felt "everything is possible".
"It is almost like 2009 where Brawn identified the double diffuser. I think there could be teams that have found loopholes, that others didn't spot, that could make the difference," he said.
"So we are taking everybody seriously. Whether it is Ferrari finishing second this year, or Williams finishing 10th. All of them could come with a car that can outperform us."
Mercedes have won every championship in the V6 turbo hybrid era that started in 2014, with Ferrari and Red Bull the only other teams to win races.
Former champions Williams scored just seven points from 21 races this season, despite using Mercedes engines, and finished last overall. The team have not won a race since 2012, with their most recent title in 1997.
Brawn, the team that was Honda and is now Mercedes, stunned Formula One in 2009 when they produced a winning car only months after being threatened with closure following the Japanese manufacturer's withdrawal. (Writing by Alan Baldwin in London; Editing by Hugh Lawson)
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