SOCHI, Russia (Reuters) - Nico Rosberg vowed to bounce back and win the next race in Texas but in his heart he knows his Formula One title hopes are over for another year after retiring from Sunday's Russian Grand Prix.
With Mercedes team mate Lewis Hamilton taking his ninth win of the season, the German is now 73 points behind the Briton with four races -- a maximum of 100 points -- still to come.
As Mercedes non-executive chairman Niki Lauda said after the race at Sochi's Olympic Park circuit, Rosberg has only a mathematical chance.
"He could have won the race today so if you lose 25 points then it’s difficult," the Austrian told Reuters when asked whether he saw any way back for the German.
"Mathematically yes, but then Lewis has to retire now and go home. He will not."
Rosberg had started on pole position and kept Hamilton behind him but his hopes of a victory evaporated early on when his car developed a throttle problem that forced him back to the pits and retirement after seven laps.
"It is tough. It really is tough," he told reporters.
"Coming here, really having an awesome weekend with qualifying, race start, first corner, everything really going to plan and then to lose it in such a way is very tough."
The retirement was Rosberg's second in four races and allowed Ferrari's Sebastian Vettel to overtake him for second place overall. The four-times champion is now 66 points behind Hamilton.
Hamilton need only score two points more than Rosberg at the next race in Austin, and nine more than Vettel, to win his third championship.
A Mercedes one-two, with Rosberg second as was the case in Austin last year, would do it.
Despite the odds, Rosberg said he would be fighting right to the bitter end.
"Bouncing back is never a problem for me," he said. "That's always worked out. I just come back, I forget the last week and learn from it.
"You just have to be realistic now, it's a lot of points. It does not change my approach. We are still pushing to the maximum, I am committed and going for it mentally.
"It's disappointing of course because I was looking to close the gap today but anyways I will go to the next race and I want to win there. That is the goal at the moment."
Editing by Ed Osmond