Changing weather disrupted the practise sessions on Friday at the Canadian Grand Prix, with Ferrari driver Fernando Alonso topping the timsheeets in the second session from Mercedes' Lewis Hamilton.
The morning session was driven under wet-dry conditions, with the second session in the afternoon, run under dry conditions, giving a clearer picture as Alonso posted the fastest time.
"It was an untypical Friday, affected by the extremely changeable weather," Alonso said. "We didn't manage to complete our programme because the track was wet in the morning and so only tomorrow (Saturday) will we find out if the updates we tested here produce a real performance evolution.
"It was very important that we try them just as it was also important to try the new Pirelli tyres. Felipe and I made different choices for the long runs and this is another reason why we will have to wait for the outcome of qualifying to understand more.
"The only certainty is that we must ensure we make the right decisions, because today (Friday) we saw how getting a clean lap depends a lot on the time you make the decision and what track conditions are like at that moment.
"We can expect a very complicated weekend, because the weather is due to remain changeable right up to Sunday and judging the grip level will be difficult, both in qualifying and in the race."
Hamilton was followed by Romain Grosjean, Mark Webber, Nico Rosberg, Felipe Massa, Sebastian Vettel and Adrian Sutil, with the Mercedes, under a cloud over unauthorized tyre testing, again looking capable of pulling off another pole position.
"Today (Friday) has gone pretty well for us and I'm pleased that it dried out this afternoon so we could get some decent running in," Hamilton said. "The track is definitely not at its full potential yet but we were able to start analysing the tyres over longer runs and work on getting them up to speed for the first lap.
"The car felt pretty good and I tried a couple of different things over the two sessions which made me feel a little more comfortable so that's positive. Hopefully it will be dry for qualifying and we can have a strong day."
To watch Canadian GP Qualifying for viewers in the U.K., click here.