It could be Ferrari, not Mercedes, presenting the biggest challenge in Canada to Red Bull's 2023 victory clean-sweep.

Fresh off the double-podium in Spain, the two Mercedes cars were first and second in free practice on Friday ahead of the Canadian GP.

But Red Bull's Dr Helmut Marko isn't too worried about them.

"They drove their times much later than we did," he said, with runaway championship leader Max Verstappen just sixth.

"The track was much better then. They also revved up their engines to the ma while we were busy with our race simulations," Marko told Servus TV.

At the same time, he admitted that Verstappen and Sergio Perez complained of "restless" cars over the bumps and kerbs.

As for Ferrari, however, having struggled so badly in Barcelona, Carlos Sainz and Charles Leclerc seem to be much happier at the Circuit Gilles Villeneuve.

"Leclerc's long run was particularly good," Marko warned. "We have to watch out for them.

"We still have some work to do on the car," he added. "The race simulations look a bit better, but Leclerc was very fast and maybe even faster than we were."

And so, Marko is warning that Red Bull's dream goal of an unprecedented clean-sweep of wins in 2023 could come to an end this weekend.

"We are all looking forward to our 100th victory, which may have added some extra pressure," he said. "At the moment it doesn't look like that 100th win is possible here, but we still have one more practice to go and hopefully we can improve the car.

"But it was surprising that the handling of the car was a bit disappointing and given what we have already achieved, we have not lived up to our own expectations," Marko added.

However, Ferrari's Sainz is not too sure he is in contention for the win on Sunday.

"I have had positive feelings in terms of pace," he said, "but the sensations are not so good with braking.

"We seemed quick, but the car was bouncing a lot and the lap on the softs was not easy."