A fourth-seeded del Potro took down top seed and former champion Jo-Wilfried Tsonga 6-4, 6-7 (9-11), 6-3 in nearly three hours on the indoor hardcourts at Palais des Sports.
Tsonga and del Potro both recorded career win number 200 on Friday.
Del Potro, who was last week's Rotterdam runner-up to the great Roger Federer, will appear in his 15th career ATP World Tour final (9-5 mark, 0-3 indoors).
The 2009 Marseille titlist Tsonga and del Potro met for the fifth time, with the towering Argentine improving to 4-1 in their lifetime series.
Llodra upset third-seeded Serb Janko Tipsarevic 6-4, 7-6 (12-10) to reach his third final here in four years.
The left-handed Llodra titled here in 2010 and was the Marseille runner-up in 2009. He will make his 10th career appearance in a title match (5-4 mark, 2-2 indoors).
Del Potro and Llodra have met twice before, with the Argentine taking both matchups.
The newest Marseille champ will collect $120,000.