Felipe Massa says a brake duct issue cost him "half a session" of running during second practice for the Malaysian Grand Prix.
The Williams driver was 15th quickest in the afternoon session and completed just 19 laps, 18 fewer than teammate Valtteri Bottas and the joint lowest amount of FP2, alongside Renault's Kevin Magnussen. Massa revealed he spent around 30 minutes stuck in the garage while his mechanics worked on the problem.
"We had some issues with the brake duct," the Brazilian explained. "We were having too much temperature at the rear on the rims so we had to stop for maybe half an hour to change the brake ducts, which was not really a great afternoon.
"It was something that was giving a lot of temperature on the rims, so we had to change. We couldn't carry on, especially for the long run, which is why I just did a quick lap on the soft tyres and then we changed.
"I would say maybe I lost half the session because of that. There is a lot we need to understand for tomorrow to make the car more competitive."
On top of his brake issues, Massa admitted Williams looks off the pace at Sepang compared to its rivals and says the team is struggling to get the best out of fresh tyres in the hot temperatures.
"Something we need to fix is we need to improve with the new tyres. On the long run we are not so bad, I would say, but with the new tyres we were not as competitive as some other cars. Driving the car was more or less OK, it's a normal feeling. There's not really anything strange or anything wrong, but definitely we need to get a better pace."