Having exited Wimbledon in the second round, Ferrer has returned to Sweden looking for a third title at the ATP 250 clay-court event to kickstart his season.
Hanescu provided little resistance on Thursday as the world number seven powered his way into the last eight with a 6-4 6-0 victory.
Ferrer, the top seed, only allowed his opponent 10 points on his serve and was helped by a dominant second set that saw the Romanian trouble the scoreboard just eight times.
Joining Ferrer in the next round are fellow seeds Pablo Carreno-Busta, Fernando Verdasco and defending champion Carlos Berlocq.
Argentina's Berlocq suffered an early setback in his match with Radu Albot before taking the match 5-7 6-4 6-2.
After a stuttering first set, Berlocq regained his strength on first serve to overpower the Moldovan qualifier, taking 76 per cent of points when he landed.
Carreno-Busta started quickly against Paolo Lorenzi by winning the opening three games, and he went one better in the second set to open up a 4-0 lead before eventually winning 6-2 6-2.
With victory in sight the Spaniard initially stuttered but after wasting seven match points he eventually sealed his progression in 73 minutes.
Verdasco had a run of six consecutive games in his match with qualifier Albert Ramos-Vinolas and he will now face Carreno-Busta after a 6-1 7-5 win.