British GP winner Andrea Dovizioso had seized the lead from pole sitter Lorenzo into turn one, but the Fortuna Aprilia star was back in front by the end of the opening lap.
Dovizioso was initially able to stick with the Spaniard, but began to decline after just a handful of laps, costing him second place to Alex de Angelis and, later in the race, third to Alvaro Bautista.
Meanwhile, de Angelis had closed to within one second of Lorenzo around the halfway mark of the 24 laps, igniting hopes of a victory battle, but the #1 soon responded and was able to keep the recurring runner-up in yet another second position by 3.857secs at the flag.
Lorenzo is now 23 points clear at the top of the standings, but with de Angelis now tied with Dovizioso for second.
Race results - Assen:
1. Lorenzo (Aprilia)
2. de Angelis (Aprilia) +3.857secs
3. Bautista (Aprilia) +8.683secs
4. Dovizioso (Honda) +17.948secs
5. H. Aoyama (KTM) +30.800secs
6. Simoncelli (Gilera) +31.135secs
7. Barbera (Aprilia) +33.323secs
8. Kallio (KTM) +40.807secs
9. Locatelli (Gilera) +1min 11.829secs
10. Takahashi (Honda) +1min 18.958secs
21. Laverty (Honda) +1lap
Linfoot (Aprilia) DNF