McLaren driver Lewis Hamilton said he was wary of the threat posed by Ferrari at the Canadian Grand Prix.
fastest in both practice sessions on Friday
in Montreal but he was only marginally ahead of Ferrari's Fernando Alonso and Felipe Massa.
"It's going to be tight, not only in qualifying but in the race," he said.
"If you look at the race pace, Ferrari look very, very quick and their tyre degradation seems to be better. We have to be careful of them."
Ferrari have turned up in Montreal with the latest stage of their upgrade programme, which is aimed at improving the performance of a car that started the season more than a second off the pace.
Despite the car's lack of performance
Alonso is leading the championship after six races,
three points ahead of Red Bull drivers Sebastian Vettel and Mark Webber, and 13 ahead of Hamilton.
The new parts on the Ferrari in Canada are revised rear bodywork and front brake disc design.
The bodywork is very similar to the layout used by McLaren, in using the exhausts to try to improve airflow and downforce at the rear of the car.
Alonso, who was just 0.054 seconds slower than Hamilton in second practice, said: "I am rather pleased with the way things went.
"The first indications seem to be positive, but it is much too soon to draw any conclusions as Friday times should never be taken at face value."
Massa added: "I think we can be happy with the fact that the updates have worked."
Hamilton was pleased with a trouble-free day during which he was quickest whenever he was out on the track.
His team-mate Jenson Button was less fortunate - a series of problems with his car meant he managed only two flying laps in the first session before two consecutive gearbox changes meant he managed only the final 18 minutes of the 90-minute second session.
But Button said he still felt he could have a successful weekend despite the time lost.
"It hurts because you can't do any high-fuel running and you can't do any set-up work," he said.
"When you change the gearbox you need to make sure it's all pointing in the right direction. We didn't do that because we had to get running.
"[But] we're positive. The car is quick, Lewis was fast. I'm not worried.
"I'm looking forward to it. It wasn't the perfect Friday but we've done worse. I have a reasonably good feeling."
"It would be nice to do some runs, we didn't really get any set-up work, I was just out there to get a feel for the circuit, but tomorrow is another day and we are still confident we can have a good weekend."
Canadian Grand Prix 2012, day two
- Saturday, 9 June: Third practice 14:55-16:05 BST, live text and audio commentary BBC Sport website. Qualifying 17:55, live text and audio commentary BBC Sport website and Q3 online and BBC Radio 5 live. Qualifying highlights 22:30-23:45, BBC One
Canadian Grand Prix 2012, day three
- Sunday, 10 June: Race 18:45-20:50 BST, live text and audio commentary BBC Sport website and BBC Radio 5 live sports extra live. Race highlights 22:30-00:30, BBC One.
Canada practice two highlights