Nico Rosberg says he cannot help but be concerned by Mercedes' unusual slump during the Singapore Grand Prix since the team remains uncertain as to why it has been struggling.

Mercedes endured a dismal weekend around the Marina Bay Circuit by its own high standards, with neither Rosberg nor Lewis Hamilton showing pace comparable with Ferrari and Red Bull.

With Hamilton going on to retire with technical issues, Rosberg crossed the line in fourth place, but while he was pleased to trim the gap to his team-mate in the overall standings to 41 points, his worry is that Mercedes doesn't ultimately understand why it suffered a two second swing in pace.

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"It is worrying because we don't know why it is like that," he told Sky Sports F1. "Who knows if it will continue like that or not. There is the small positive of closing the gap, but the big one is that it has been such a disappointing weekend for all of us.

"To be so far off the pace all of a sudden and not understand it, that is really bad, because then how do you improve it? The next track we just hope will come towards us and the chances are extremely good because at all other tracks we have been so fast, but who knows?"

With the next event in Japan starting in just four days time, Hamilton remains confident that the issues experienced in Singapore are merely a blip.

"Our reliability is really high as we had an amazing season so far. It's still a long way to go and I know I lost some points today, but I was fast and on form and I will make sure I bring that out to Suzuka to fight back."