Lewis Hamilton will start tomorrow's race from fourth place on the grid, which is not where the reigning champion expected to be.
Mercedes has been at the front of the grid for the last six Chinese Grands Prix, with Hamilton himself on pole for four of those occasions.
However, this time the Silver Arrows never looked like seriously threatening Ferrari's dominance of the front row.
"I'm not really sure," Hamilton admitted when asked what the reason had been for Mercedes' drop in competitiveness this weekend. "The car felt okay, but I don't really have the answer for it."
Team boss Toto Wolff was likewise struggling to explain the turnaround in form between Mercedes and Ferrari. He had also been taken by surprise by Ferrari's surge in pace on Saturday in Shanghai.
"They've been really strong all day, already in the morning," he told Sky Sports F1. "But qualifying performance, they put one on top. We really have something to think about.
"It's tricky," he said, suggesting that optimising the tyres was proving problematic. "We're lacking grip. You can fall out of the window by the tyres getting too hot, or by the tyres being too cold. The two extremes like we had in Bahrain.
"I think this is what happened," he said. "I think it's a tyre issue.
"Tomorrow it's expected to be much warmer," he pointed out. "I hope we've done the right thing set-up wise and that we have better pace in the race than Ferrari.
"We just need to have pace. And if you have pace, you can undercut the overcut!"
Currently 17 points behind Vettel after the first two races of 2018, Hamilton can ill-afford to drop further behing Sebastian Vettel in the title standings.
However, he was taking some comfort from the knowledge that it was easier to overtake at Shanghai than many of the circuits on the calendar.
"It's in the top three of the easier ones to overtake," he agreed. "So I think the delta is generally a second. But the Ferraris are too fast on the straight, so it will be interesting.
"We're half a second behind today, so honestly I can't tell you if we can challenge," he said. "We were quicker in the race in the last race, but they were able to hold on. They'll probably do the same tomorrow.
"It's a lot hotter tomorrow, so it will be a real challenge to see if we can keep up with them," he added.