Vettel should have moved 21 points ahead of Hamilton at the German Grand Prix.
Instead he heads to Hungary this weekend 17 points behind the Briton after his dramatic retirement at Hockenheim on Sunday.
Vettel crashed out in the slippery conditions and, to make matters worse, title rival Hamilton sealed a famous victory from 14th on the grid.
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“It was surely one of the darkest moments of his whole career”
“What an unbelievably dark day for Sebastian,” said Rosberg, who had a stormy relationship with Hamilton when the pair were team-mates at Mercedes.
“It was surely one of the darkest moments of his whole career.
“It was his home race, he had the chance in his hands to win and take a big lead in the championship, but he chucked it away in the wall.”
Vettel has been found wanting in the heat of his championship battle with Hamilton.
His mistake on Sunday was his seventh high-profile mistake in a little more than 12 months.
During the same period, Hamilton has not made a significant error, and it is proving the difference between the two quadruple world champions.
But despite Vettel’s latest woe, it still required all of Hamilton’s brilliance to claim the 66th win of his career.
“Never write off Lewis because he always comes back, and that is a fact,” added Rosberg. “He showed it again on Sunday by taking home the win.”
/ Motorsport – ENG Portals