Jenson Button blamed unexpected rain during the final minutes of Q3 for his fifth place on the grid for the Belgian Grand Prix.
Button had expected to qualify on the front two rows, but was caught out by a damp track surface at La Source late on after saving his only set of fresh option tires for his final run.
"We only had one set of the new option tires left, so we chose to use them for the last run in Q3," said Button. "Unfortunately it started to rain - which we hadn't expected - and that cost me 0.3sec in the first sector. It was time I managed to make up in the second and third sectors, but we're fifth.
"It's a shame, because two of the cars ahead of us did their best laps at the start of Q3 when the track was at its best. We didn't have an extra set of new tires, so we started Q3 on scrubbed options instead.
"It was a very tricky session, especially because on this asphalt it's very difficult to work out how much water is on the circuit and what the grip levels are."
Despite qualifying behind his championship rivals Mark Webber, Lewis Hamilton and Sebastian Vettel, he remains confident of a good race, especially in light of his wet wins in Australia and China earlier in the year.
"The performance is there in the car and I'm sure we can have a good race," he added. "I'm fifth, and although I'd have liked to be a bit higher up, that puts us in the mix."