Bottas has to challenge for the title - Wolff

Mercedes boss Toto Wolff has insisted that Valtteri Bottas has to challenge for the 2018 title up against his team-mate, Lewis Hamilton, next season, and has backed the Finn to improve on a mixed maiden season with the Silver Arrows in the 2017 campaign.

Wolff's outfit won their fourth championship in a row last year and Hamilton also claimed his fourth driver's title, but Bottas ended up finishing in third behind Sebastian Vettel in second and 60 points behind his team-mate in first.

The Mercedes boss has backed Bottas to improve on his three Grand Prix wins from last season which took place in Russia, Austria, and Abu Dhabi.

"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 the 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," Wolff added.

"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."

Mercedes won the team championship in 2016 and the driver's title also through Nico Rosberg who had a tumultuous relationship with Hamilton both on and off the track. After winning the championship, he claimed the key to beating his Silver Arrows team-mate was to mess with Hamilton's head, but Wolff does not think Bottas will adopt this tactic.

"That's not at all Valtteri's style. Valtteri wants to do it on the track in the best possible way and he has driving skills that have worked out for him at certain circuits where he was the one that set the benchmark. Valtteri will improve his weaknesses and continue to work on his strengths.

"What he wants is to be benchmarked against a four-time world champion. He's brutally honest with himself, if he can step up and challenge Lewis he has his place among the greatest drivers in Formula One. If he doesn't he's going to know it and there is no excuse."