Red Bull believes Daniel Ricciardo won the Monaco Grand Prix with a car that was 25% down on power, after suffering an MGU-K failure early in the race.
Ricciardo made it clear over team radio he was suffering a lack of power from his car - which allowed nearest challenger Sebastian Vettel to close up.
Speaking after Ricciardo held on to take his second win of the season, Red Bull team boss Christian Horner revealed more details about the problems.
"He has lost about 25% of the power of the engine," Horner told Sky Sports.
"And then because of the way these engines work, his rear brake temperatures are going through the roof.
"We saw it with Lewis Hamilton and Nico Rosberg a few years back in Montreal.
"He is having to cool the brakes, he is having to cool the car, he is having to lift off to do that.
"He is having Sebastian Vettel breathing down his neck. He cannot make a mistake. He cannot lock a wheel up.
"He is dealing with all the switch changes and all the stuff that he has got to manage. And he totally nailed it. He was the coolest guy out there today."
When an MGU-K fails, it not only results in a loss of the 160hp power boost from that system, but it also has an impact on brakes too as the MGU-K is charged by kinetic energy off the rear brake-by-wire system, so when it fails that resistance is gone - which means the rear brakes have to work harder and risk overheating.
Ricciardo thought it was over
Ricciardo said when the problem first struck "I went on the throttle and had what felt like half the power".
"I expected my race to be over in a few corners," he added. "After a few laps I was stressing out and the team said 'this is you for the rest of the race now'.
"I still don't really know how we did it, obviously it helps that the track's so difficult to overtake.
"That was not a good feeling and I couldn't really enjoy much of the race after that."
Horner said there were discussions in the Red Bull pit about Ricciardo having to retire, but it was decided against because he was still in the lead.
"It was unbelievable," Horner added. "He was not going to give this race up, this weekend. He has been quickest in every session.
"We lost the MGU-K 17-18 laps into the race, and that is 2.5 seconds per lap he is giving up.
"Then your brake temperatures go out of control, the fuel [consumption], tyre temperatures go up - and he just managed it like he was on a Sunday afternoon drive.
"They are telling me on the intercom that we are going to have to retire the car after one or two laps, and I said look, we are in the lead of the Monaco GP, we are keeping going."