Jenson Button admitted he was mystified by his lack of speed during qualifying for the United States Grand Prix.
The Briton was knocked out in Q2 and qualified in 13th place, but will start from 16th position after being hit with a penalty for overtaking under red-flag conditions on Friday.
Button finished six places behind McLaren team-mate Sergio Perez and said he expected much more.
"I haven't got a clue," said Button when asked about his lack of pace. "It's strange, because all weekend I've been very quick.
"I didn't go quick enough, basically. I don't know why."
He admitted the conditions, warmer and windier than on Friday, had made for a strange session.
"It's very weird, especially in Q1 when you see [Valtteri] Bottas quickest and his team-mate out. I don't get it," he said.
Perez will start from seventh position, the Mexican bouncing back from a difficult Friday during which he struggled with a car suffering a mechanical problem that was discovered overnight.
"It was better than expected. I'm happy to see the car has improved. We did a good job," said Perez.
"Tomorrow I have nothing to lose and I hope to give the fans a great race.
"Unfortunately the work we did yesterday is not going to be useful because the car had a problem, so it's going to be interesting to see what I can do."