Nico Rosberg described his sixth place on the Monte Carlo grid as a "terrible" performance, as he felt his Mercedes had been quick enough for pole position.
The German was fastest in Q2, and was devastated that he could not maintain that form.
"I'm very sorry, I'm in a bad mood because I did terribly," Rosberg said to television reporters. "I should have had pole or at least first row of the grid today because in Q2 it was easy for me to be first with just one set of tires so I should have been on the front row or just right up there."
He said there had been no specific problem other than a failure to get the best out of the car and find gaps in the traffic.
"It just went completely wrong in the last qualifying -- just didn't get it together -- too many laps with traffic and just made a mistake myself," Rosberg said. "Just a disaster and very, very disappointing. So I'm really sorry to the whole team that it went so bad. Difficult race tomorrow, then, starting sixth."
Michael Schumacher will start a place behind Rosberg in seventh in the second Mercedes.