Valentino Rossi, looking to end his longest losing streak in the premier-class, was instantly on the pace at Sepang in Malaysia on Friday.
Riding at a circuit where he had dominated in pre-season testing, the Fiat Yamaha rider took over at the top from Ducati's Casey Stoner shortly after the Australian fell and - barring a brief interruption by Andrea Dovizioso - remained fastest for the rest of the hour.
Rossi finished the session 0.109sec in front of the Repsol Honda rider. The Italian's team-mate and title leader Jorge Lorenzo, with whom Rossi battled forcefully for third last weekend in Japan, was in third place (+0.174sec).
"It's been another good start for us here and we found a good setting straight away," said Rossi. "I am feeling very comfortable on the bike and so far my shoulder is okay here.
"The track is quite dirty but that will get better and I love riding at this track, it's one of my favourites and I am always happy to be on the bike here.
"We are confident that we will be able to make some more improvements tomorrow and be in good shape to try for the front row tomorrow afternoon."
"It's nice to start the weekend at the front again and this was a good session, with Valentino fast from the beginning," added team manager Davide Brivio. "It seems he has a good pace here but of course it's only the first session and we have more work to do.
"The track isn't in great condition; it's quite slippery at the moment but this will improve throughout the weekend. Tomorrow we will work on our Bridgestone tyre choice and fine-tune the setting ahead of qualifying."
Rossi, who broke his leg at round four, hasn't won a race since the Qatar season opener.