Oct.22 (GMM/Inautonews.com) Nico Rosberg is refusing to give up on the 2015 drivers’ championship, although even his Mercedes boss does not agree.

“It’s a shame that Nico was not able to keep fighting for the title because he has done very well,” Toto Wolff told the Spanish daily Marca ahead of the US grand prix.

The Mercedes chief admitted reliability has thwarted Rosberg’s chances, warning that even second place for the German is now in doubt.

“Second place is in danger,” Wolff admitted, “as Sebastian (Vettel) is now ahead, but Nico has had very bad luck.

“It’s not finished, we have four races to go and in Sochi he (Rosberg) was very strong with pole position and leading the race until the car broke down.

“The results we have seen (from Rosberg) are below his actual level.”

Rosberg acknowledges that his 73 point gap with just 100 points left to win is a major hurdle, but he refuses to give up.

“I’m still fully focused on it and there are still four races,” he told the German broadcaster RTL in the days before Austin.

“We have an amazing car and I go to the last four races trying to win them. That’s my aim,” he said.

But even if Rosberg does complete the season with a winning spree, Hamilton would have to strike an equally-unlikely patch of terrible luck.

Indeed, in Austin, Rosberg’s quest will end if he scores just 2 points fewer than Hamilton, while Vettel’s mathematical chances will be over if his deficit in Austin is more 9 points or more.

It would complete the back-to-back clean sweep for Mercedes of two drivers’ and two constructors’ world championships in 2014 and 2015.

“Yes, the great triumph is close,” agreed Wolff, “but it is not done yet. 66 points is a lot, but there are still 100 in play.

“We’ve seen great comebacks before in F1, so I don’t like to start celebrating until it is actually done.

“This is always the danger of sport — you always need to be awake and ready to react. Formula one is a very rapidly changing world,” explained Wolff. “You always need to be careful.”

For instance, the weather specialists Ubimet have announced that Austin is currently bracing for “the biggest rain event” seen for months in Texas, which “will affect” the US grand prix.