Nico Rosberg described Sunday’s Singapore Grand Prix as the toughest day of the season to date after an electrical failure saw him relinquish the championship lead to team-mate Lewis Hamilton.

The Mercedes team confirmed a faulty wiring loom in the steering column was to blame for Rosberg’s second retirement of the season, leaving the German powerless to defend his points lead.

Rosberg first encountered the problem when he left the garage with the issue which then forced him to start from the pit-lane after being stranded on the grid during the formation lap.

Shifting issues and the lack of ERS eventually put him into retirement after just 15 laps.

To make matters worse for Rosberg, arch rival Hamilton came through to take victory, turning a 22 point lead into a three point deficit in the championship standings.

The combination of the wiring loom failure and the loss of the championship lead left Rosberg to describe the experience as his low point of the season so far.

“(It started) as I got in the car in the garage,” he said.

“They’d (mechanics) sat in the car five times just before I got in, doing all sort of checks, everything was OK, then I get in the car and it didn’t work anymore – which is crazy.

“The dash was working, gearshift paddles were working, and that’s it. No clutch, nothing else – just gearshift, dash and lights.

“It’s strange, some things were working, some things weren’t.

“Until they switched my car off and pulled me in the garage I still believe I could have a good race.

“At the moment I’m just disappointed. I want to begin to understand with the team what went wrong.

“So that was all part of the day: the toughest day of the year for me.”

It is understood the Mercedes squad are due to send the faulty parts back to its Brackley base for full analysis before the Japanese Grand Prix.

Although delighted with victory, Rosberg’s team-mate Hamilton admitted the team still have plenty of work to do in terms of reliability following this latest mechanical failure.

“I know that the team will not be 100 per cent happy because we want to win collectively,” said Hamilton.

“We want to get those one-twos, we want to be the dominant team all together, so by not getting that result, they’ll be going back to the drawing board trying to figure out what happened.

“They’re constantly coming up to things and perhaps other people are starting to be a bit more reliable than us so that’s an area that we can still definitely improve on.”