Felipe Massa labelled a "very difficult" day as he struggled to get the tyres into the correct operating window during practice for the European Grand Prix.
Williams team-mate Valtteri Bottas was fourth quickest at the Baku City Circuit but Massa could manage only the 16th fastest time, 1.3s adrift of the Finn.
Massa pinned the deficit on getting Pirelli's tyres to work correctly, an issue which he says is new for him this season.
"It was a very difficult day for me," he conceded.
"I was struggling to get the tyres to work and I’ve never had a problem like this so far this season, so it’s something that we need to analyse.
"I couldn’t drive the car how I would like because the tyres weren’t within the working range, so whenever I was out on track I was struggling massively.
"All the other drivers were learning a lot and improving every lap but I couldn’t.
"There’s a lot of work to do before tomorrow, but I’m sure when we have looked at everything and made some changes, I’ll be competitive."