Nico Rosberg was disappointed after retiring with a mechanical problem while running in sixth place at the Malaysian Grand Prix.
The Williams driver, who finished in seventh place in the season-opening race in Australia, was looking set for another strong result when he stopped with smoke coming from the back of his car with 12 laps to go.
The team were unsure what the cause of Rosberg's retirement was.
"We managed to improve our qualifying performance from Australia so I had a good starting position but I lost sixth place to Kubica at the start," said Rosberg. "Later he made a mistake or had a problem and I thought, I'm right next to him and I managed to claw the place back.
"I had a good run through the next couple of corners to hold the position and that opened up my race really well. From there I gave it everything I had, but it all came to a halt. It was a great shame not to finish but things are progressing and we hope we can do better in a week's time."
Williams failed to score any points at Sepang, despite teammate Alex Wurz enjoying a strong run from the back of the grid.
The Austrian, who was hit by a gearbox problem in qualifying, climbed up to ninth position in the race.
"It was a good race and I had fun, especially in the first stint, when I had a good pace on soft tyres and could overtake," said Wurz.
"I am also happy from the physical point of view, because I feel ok, even after such tough afternoon. Starting from 19th made it difficult to fight against Trulli and Kovalainen by the end of the race and it was disappointing to finish just out of the points.
"I now look forward to Bahrain, as our car is now much better in qualifying and I hope that I can extract there the same race pace I had here to in order be able to bring some points to the people at home who are working very hard."