Felipe Massa has said that he is unable to determine why his Ferrari struggled for grip during qualifying for the United States Grand Prix.

The Brazilian registered the sixth-fastest time in Austin, but said that he was surprised by the lack of traction and is unsure if it is a problem with the car or the tyres.

"I don't know what's happened, I'm very surprised," AutoSport quotes Massa as saying.

"I'm struggling to get grip from the car everywhere and it's difficult to explain why.

"I've no idea what's going on and we need to find out whether there's a problem with the car or the tyres."

Lewis Hamilton has also voiced his concerns over the poor performance of his tyres.