Lewis Hamilton said he was at a loss to explain why he could qualify only 14th on the grid for the Monaco Grand Prix.
The Mercedes driver will start 13th after penalties for other drivers but faces major damage to his title hopes with rival Sebastian Vettel's Ferrari on the front row.
Asked to explain why he was so slow, Hamilton said: "I can't at the moment. I did everything I could in the car.
"It is going to be a real struggle to get into the points."
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Hamilton added that he was "devastated, to the point that I could not get out of the car" when he returned to the pits.
He said the problem was "a tyre issue - I didn't get the grip from the tyre".
He added that it was "similar problem" to the Russian Grand Prix a month ago, when the three-time champion was also off the pace because of overheating tyres.
"Other people are making them work," he said. "We just have to get our head around it. It's a mystery. None of us can understand it."
Hamilton had two out-of-control moments during the second session when he just managed to avoid a crash.
His final attempt to get into the top-10 shootout was thwarted when McLaren's Stoffel Vandoorne crashed but the 32-year-old felt it made little difference.
"I think I would have struggled," Hamilton said.
"That lap may have just got me into the top 10 and then I probably would have struggled to be in the top five with whatever issue I was having with the car."
Mercedes team-mate Valtteri Bottas is third on the grid, just 0.045 seconds slower than the time set by Ferrari's Kimi Raikkonen to take pole position.
Hamilton said: "It's great to see Valtteri was able to extract one sense of what the performance was.
"It shows we were not terrible here and we will just have to figure out why I was not able to be up there with him."
Mercedes non-executive director Niki Lauda said: "I cannot explain because we do not know yet. We have to analyse everything. Both cars were the same, Bottas' car worked fine and his [Hamilton's] did not.
"I feel terrible for Lewis, he certainly knows how to drive around Monte Carlo, so it is not his fault. But nevertheless we have to find out the reason."
Bottas said: "Ferrari definitely were quicker today. They have been strong all weekend and we have been struggling a little with the set-up.
"The car seems very sensitive to set-up changes. In second practice we were way off, we got it better through P3 and qualifying but it was still not an easy car to drive.
"But I felt I was able to get more confident in qualifying and extract more and the last lap was mega - the best I have ever done around here.
"Pole was 0.05secs away, but there was no more in the car, I tell you. We thought Ferrari would be further away, at least 0.2secs.
"It's a shame they locked out the front row because now they have the upper-hand with strategy and everything. But it will be an extremely long race here, we look for opportunities."