The Yamaha rider, who leads the standings thanks to Casey Stoner's dramatic last-lap fall in Germany last weekend, was dominant throughout the two sessions, setting his best time of one minute 48.076 seconds early in the afternoon running.
That was enough to edge out Dani Pedrosa by just three hundredths of a second, the Spaniard having been boosted by his win in Germany and the news that he will remain with Repsol Honda and ride alongside Moto 2 prodigy Marc Marquez next season.
Nicky Hayden gave Ducati an excellent opening to their home race weekend by setting the third fastest time, with Hector Barbera fourth on the customer Pramac machine.
Andrea Dovizioso was next up on the first of the Tech 3 Yamahas, but it was a disappointing day for Stoner, who could only manage sixth quickest time, four tenths down on title rivals Lorenzo and Pedrosa.
The Australian's cause was not helped when he was held up by traffic on a flying lap, with an error costing him during a separate effort at a quick time.
Ben Spies and Cal Crutchlow were seventh and eighth but Valentino Rossi's travails on his Ducati continued as he languished down in ninth.
Stefan Bradl was 10th with Alvaro Bautista 11th on the Gresini Honda.
Bautista's machine has run in black livery so far this season as a mark of respect for Marco Simoncelli following the Italian's death at October's Malaysian Grand Prix.
But the team have returned to their white colour scheme after being encouraged to do so by Simoncelli's family.
A delighted Lorenzo admitted his YZR-M1 had been a joy to ride.
He said: "I'm really happy because from the beginning we've been quite fast. We haven't had to make many changes to the bike, the feeling is great with the track and it's working well.
"I feel good with the suspension, throttle and braking. Everything seems quite clear, we have to work on some small details but we've made an excellent beginning."