Nico Rosberg has revealed that an ERS error was key to allowing Formula 1 title rival Lewis Hamilton to overtake him for victory at the United States Grand Prix.
The German led the early stages of the race from pole position, but he was unable to hold on to the front spot after Hamilton pulled off a move at the end of the back straight on lap 24.
Following a post-race debrief, Rosberg discovered that he had been left without extra ERS power to defend the position after requesting the boost via a switch change rather than using a button.
"I made a mistake because I used the wrong thing to get more KERS," said Rosberg, when asked by AUTOSPORT about the move.
"When I saw him coming I decided to go for extra boost but I never got it.
"I thought I was using the right thing, but there was a delay in that one.
"If I did it with a button it would be immediate, but with a switch there is a delay, so I never got the extra KERS.
"I had a big drop off, which is why in the last metres he got a good jump on me.
"So, that was a mistake that I didn't know about."
Although Rosberg accepts he could have been more aggressive in defending the inside line against Hamilton, he thinks it ultimately would have made little difference.
"I am sure I could have done better, but it is a judgement thing," he said.
"If I defended a lot he would brake late on the outside and get me on the next one.
"I thought indicating I am closing the door would have been enough to stop him from trying, but it wasn't.
"Because I was part way over already, I thought that was enough for him to think he should not try, but it wasn't."