Valtteri Bottas, Mercedes’ second driver, was called up as a late replacement last year after Nico Rosberg’s shock retirement.
And the Finnish driver had a strong maiden season by finishing third in the Drivers’ Championship.
But Wolff is piling the pressure on Bottas and claims he “has” to challenge Hamilton for the title.
It is unlikely Bottas will get anywhere near Hamilton, who has four world titles to his name.
And it adds to speculation that Mercedes are wanting to dump Bottas when his contract expires at the end of the year.
"That's what he has to do [challenge for the title]," Wolff told ESPN.
"We have seen some highlights in Sochi and in Austria and then after the summer Lewis came back very strong and the step that Lewis made up, Valtteri went backwards.
"He recovered at the end of the season and he is a Finn with a strong never-give-up mentality - Sisu - and I expect him to come back strong with all he learning he has done during the season against the best Formula One driver of the current day and age.
"I think he's the first one who wants to prove it to himself.
"He knows the deficits he had during the season and he knows where he had advantages.
"Valtteri, I've known for ten years and ten years ago a little kid showed up in the winter in my office with a little pullover, no jacket, no father with him, and he has a super-strong will, an iron will and character, and I have no doubt we will see a better Valtteri in 2018."
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