Jack Miller led reigning MotoGP champion Marc Marquez by more than one second to top a drying first practice for the French Grand Prix.
The new Le Mans track surface gradually dried on Friday morning following rain on Thursday, a process not helped by cool and predominantly cloudy conditions, while more rain is forecast for the afternoon.
Times improved through the first half of the session, with Miller, Jorge Lorenzo and Marquez the first riders into the 1m43s, 1m42s and 1m41s barriers respectively.
The 1m41.837s of Honda's Marquez was the best entering the final 10 minutes, as riders first tried slicks.
With slicks on his Marc VDS Honda, Miller reclaimed top spot with a 1m41.599s, then lowered the benchmark to 1m40.173s and 1m39.328s on his next two laps.
That meant Miller was 2.509 seconds clear at the top with seven minutes to go, and after two more improvements he was down to a 1m38.204s to lead the way by 3.103s with three minutes remaining.
Loris Baz and Marquez got that margin closer to two seconds again, before Miller went even faster with a 1m37.467s that would not be bettered.
His advantage of 3.019s was cut initially by Cal Crutchlow and then Marquez, with Miller's eventual margin 1.288s.
Re-signed by Tech3 Yamaha for 2018 on Thursday evening, rookie Johann Zarco slotted into third place, bumping LCR Honda's Crutchlow down to fourth.
Avintia and Aspar Ducati's Baz and Karel Abraham were fifth and sixth on their 2015-model Ducatis, ahead of KTM's Bradley Smith, who sat out most of the session.
Valentino Rossi spent the bulk of the session well down the order, occupying 22nd place with five minutes to go.
However, a series of late improvements meant he finished eighth, two spots ahead of Yamaha teammate Maverick Vinales, who had a brief spell at the top early and was even second to Miller at one stage with five minutes remaining.
Jerez winner Dani Pedrosa was 13th, with Ducati pair Andrea Dovizioso and Lorenzo 14th and 16th, the latter having a lap time cancelled late for exceeding track limits at the final corner.
Making his first MotoGP appearance since the 2011 German GP, standing in for Suzuki's injured Alex Rins, Sylvain Guintoli wound up in 20th place.