Defending constructors' champions Red Bull admitted on Friday that they face a huge challenge in their attempt to make an impact at this weekend's Italian Grand Prix.

Both Mark Webber and defending drivers champion Sebastian Vettel said they were struggling to stay in touch with their rivals in Friday's practice sessions.

Webber was ninth fastest in the morning's opening free practice session, but it was the team's only top-10 finish of the day.

The Australian came in 11th in the afternoon, while German Vettel was a lowly 13th.

The team suffered similar problems in last weekend's Belgian Grand Prix and are working hard to overcome the difficulties in order to be competitive on Sunday.

"We are working hard, but we are not quite there yet," said Webber. "We were a little bit more competitive on the long run when we have got a bit of fuel in, but we have got a bit of work to do with the fuel out.

"We are not normally Friday world champions, but we have a bit of work to do tomorrow (Saturday) to make sure we can make it all up."

Vettel added: "I think overall we are not as quick as we would like to be, so there is still some work to do I would guess."

Vettel added that he was also uncertain about his car's potential for Sunday's race with full fuel tanks.

"I don't know what Mark did, or what the others did, but I think we are probably more competitive in race trim," he said.

"But for sure today wasn't as good as we wanted our true pace to be, so you can see there is a lot to do.

"So far, we ran through our programme and we didn't have any problems. There is a lot to go through and learn from so we have to see."