Starting fourth on the grid, Lorenzo moved through the field to win his fourth race in six this season and third in a row.
The Spaniard now has a 25-point lead in the overall standings over Australia's Casey Stoner, who finished second more than three seconds behind Lorenzo.
Lorenzo, who this week signed a new two-year contract with Yamaha, says “This season is just perfect.”
Lorenzo dropped to sixth after a poor start but went in front for the first time at the halfway point. The lead changed hands five times in half a lap before Lorenzo pulled away decisively.