Force India's Paul di Resta set the pace in a wet-dry first practice session at the Canadian Grand Prix.
In a brief period of running with dry-weather tyres on a still-damp track, the Scot was 0.088 seconds faster than McLaren's Jenson Button.
Lotus drivers Romain Grosjean and Kimi Raikkonen in third and fifth places, sandwiching Ferrari's Fernando Alonso.
Mercedes' Nico Rosberg was fastest through most of the wet running but ended up seventh.
His team-mate Lewis Hamilton was down in 16th.
Di Resta said: "I was definitely very cautious in the first sector because there were some yellow flags. I had a good run on inters and I've been happier with the car than in the past.
"We have carried on from the good performance in Monaco. In both conditions the car is working well."
Williams development driver Susie Wolff, in her role as BBC 5 live analyst, said: "Running on intermediate tyres is always useful, but at the end it was all about who got the lap in on the drier track."
Toro Rosso's Daniel Ricciardo ended up sixth fastest ahead of Rosberg, McLaren's Sergio Perez and the Red Bulls of Sebastian Vettel and Mark Webber.
Despite the tricky conditions on a demanding track, the only significant accident involved Williams's Pastor Maldonado.
The Venezuelan lost control at Turn Three and smashed head on into the concrete wall on the outside of Turn Four, damaging the car's front wing and nose.
It was one of a series of modified wings Williams have brought to this race, all based on last year's front wing, which the team hope will lead to a step forward in performance after a disappointing start to the season.
Mid-way through the session, Hamilton narrowly avoided a collision when Ferrari's Felipe Massa slowed on the back straight before coming into the pits. That forced Red Bull's Mark Webber to slow behind the Brazilian, and Hamilton had to take to the grass to avoid them, cutting the final chicane.
The wet weather meant very few drivers ran the development tyre Pirelli have brought to Canada with a view to using it for the rest of the season.
So far, Ferrari, Lotus and Force India have blocked the introduction the tyre, which Pirelli want to use to prevent failures, because they are concerned it could put them at a competitive disadvantage.