The Aprilia rider beat Spain's Marc Coma (KTM) and Argentine Javier Pizzolito (Honda) on the 57km special stage between Necochea and Energia in Argentina.
'Chaleco' Lopez negotiated the dunes along the Atlantic Ocean south of Mar del Plata in a time of 32 minutes 37 seconds, 14 seconds ahead of Coma and 27 faster than Pizzolito.
Coma, winner in 2006, 2009 and 2011, is expected to fight for victory on the 14-stage race with Frenchman Cyril Despres, who was 1:48 in arrears of Lopez and 13th on the stage.
The stage was marred by the death of Argentine rider Jorge Martinez Boero just two kilometres from the finish.
Following the special section, the competitors have 600km to bivouac to reach Santa Rosa de la Pampa.
The rally is held until January 15 and moves into Chile before ending in Lima, Peru.