Paul di Resta emerged at the top of the timesheets as a damp-but-drying first practice session for the Canadian Grand Prix saw the order change in the final few minutes.

Persistent rain before the 90-minute session got underway ensured a wet track greeted the 22 drivers - including American Alexander Rossi taking the place of Charles Pic at Caterham - for the opening salvo of the weekend, but it was the usual names at the top of the order as Nico Rosberg and Sebastian Vettel traded early times.

While the Germans held sway for much of the session, however, it became clear that whoever timed their run just right on slicks would have the best chance of emerging at the top of the pile when the chequered flag fell.

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With Pirelli providing a new 'development' tyre for opening practice, it was somewhat ironic that di Resta should be the one to post the fastest time, as his Force India team is among those attempting to block the move to introduce the rubber on safety grounds for the remainder of the season. The tyre had originally been planned as the Montreal race rubber, but that decision was reversed in the wake of the controversial Mercedes test in Barcelona and the tyre will make now full competition debut at Silverstone later this month.

The man di Resta toppled off the point was countryman Jenson Button, who had underlined McLaren's favourable wet weather performance by vaulting to P1 as the session entered its closing phase. Also running the 'development' slick, the 2009 world champion ended the morning less than a tenth off the pace.

Joining the two Britons in the 1min 21s were Romain Grosjean, Fernando Alonso, Kimi Raikkonen and Daniel Ricciardo, although both the Finn and Red Bull's Mark Webber could have expected to improve further had they not been fractionally too late to complete one final lap. The Australian duly wound up tenth overall, just behind RBR team-mate Vettel, but also trailing Rosberg and Sergio Perez, who filled out seventh and eighth places.

With only a handful of drivers getting a lap in on the slicks, there were some big names a little further down the list than expected, with Felipe Massa eleventh and Lewis Hamilton 16th. The Briton ended his morning just three-tenths clear of Nico Hulkenberg's Sauber, which was slowed by a KERS failure, while both Giedo van der Garde and Max Chilton got into the similar 1min 25secs bracket.

Rookie Rossi, who is due to fly to Le Mans at the end of the day in order to test ahead of his race debut in two weeks' time, was a solid 20th overall, ahead of the session's two biggest victims. Pastor Maldonado spun out at the chicane, removing the nose from his Williams, while Jules Bianchi brought up the rear after being told to switch off and park up less then 20 minutes in.