It was the Spaniard who made the breakthrough in the fourth game to take a 3-1 lead in the first set. He comfortably served it out, but had the chance to break the Argentine to claim another valuable game in the race for qualification from this pool.
Del Potro managed to reverse the tide in the second set as this time he broke serve for a 3-1 lead.
Both men struggled to create opportunities on each others' serve, and this time the 2010 US Open champion served out to set up a deciding set.
Ferrer seized the initiative straight from the off, quickly building a 3-0 advantage. Del Potro weathered the storm and got back on serve at 4-3, but found himself three match points down when serving to stay in the match.
Del Potro fired a forehand long at the first opportunity to give Ferrer the vital victory.