- Red Bull's Sebastian Vettel leads practice ahead of the Italian Grand Prix
- The triple world champion leads Ferrari's Fernando Alonso by 46 points
- Vettel's teammate Mark Webber was second fastest, with Alonso fifth
- Mercedes' Lewis Hamilton says his car is not quick enough
Red Bull's Sebastian Vettel urged caution ahead of this weekend's Italian Grand Prix, despite a dominating performance which underlined his position at the top of the world championship standings.
Formula One's triple defending champion went fastest in practice ahead of Sunday's race at the Monza circuit with a lap of one minute and 24.453 seconds, over a half a second clear of his teammate Mark Webber.
Vettel, who is 46 points clear of second-placed Fernando Alonso of Ferrari in the drivers' championship, expects a keen battle for pole position in Saturday's qualifying session.
"It was a good day, but it is still only a Friday," the German, a two-time winner of this race, told reporters.
"It looks tight. If you look at it too much it can be a distraction, but I think it will be a close qualifying with many cars up there.
"Tomorrow, I expect it to be close. It is a short lap and there are not too many corners -- so not that much you can do right, and not much you can do wrong."
Red Bull announced earlier this week that Daniel Ricciardo, currently driving for Red Bull's sister team Toro Rosso, will replace Webber in 2014 when his fellow Australian quits F1 to race in the Le Mans series for Porsche.
"Although we're quite competitive, we're not quick enough yet and will have to work hard tonight to see where we can make some improvements," said Webber.
"Of course we don't know what fuel loads other teams are on so we'll have to wait and see but there is a bit of work for us to do if we want to compete right at the front tomorrow."
Ricciardo, however, was off the pace -- 13th in the first session and 17th in the second.
"Unfortunately, we seem to have gone in the wrong direction," the 24-year-old said. "Having run only the hard (tires) in the morning, in the afternoon, I did not set a competitive time on the medium.
"I did make some mistakes and I will admit to that, but there were too many of them so we definitely need to get the car to suit me a bit better. At the moment, it's on a knife edge."
Lewis Hamilton won this race with McLaren in 2012, but a repeat performance with his current team Mercedes looks unlikely.
The 2008 world champion could only go sixth fastest in the afternoon session after setting the pace in the morning, just ahead of seventh-placed teammate Nico Rosberg.
"Although we're quite competitive, we're not quick enough yet and will have to work hard tonight to see where we can make some improvements," said the Briton.
"Of course we don't know what fuel loads other teams are on so we'll have to wait and see, but there is a bit of work for us to do if we want to compete right at the front tomorrow."
All eyes will be on Alonso at Ferrari's home grand prix. The Spaniard registered the fifth fastest lap time, behind the Lotus duo of Kimi Raikkonen and Romain Grosjean.
Alonso's teammate Felipe Massa was eighth fastest, with McLaren's Jenson Button and Sergio Perez rounding out the top 10.