Despite dominating possession in the first half, the hosts put in a toothless display as they struggled to break down their endangered opponents’ compact back five.
Chances were subsequently few and far between, although one fell to Vitinha, who brilliantly brought down a long ball over the top before wildly shooting well off target from inside the area.
That lack of a cutting edge came back to haunt Marseille, as Auxerre took the lead with a real bolt out of the blue thanks to Birama Toure’s stunning strike, as he guided a sensational volley into the top corner from 25 yards after Pau Lopez’s punch fell his way.
Marseille looked to hit back before the break, but for all their pressure, the closest they came to an equaliser was through a couple of goalmouth scrambles that were eventually cleared.
Marseille coach Igor Tudor brought on Nuno Tavares after half-time and the hosts immediately carried more menace, finally forcing their first shot on target through Vitinha.
Tavares then had a fierce effort from close range palmed around the post, before Chancel Mbemba headed harmlessly over the bar, as the home crowd grew evermore restless.
However, Auxerre’s resistance was finally broken by Under with 15 minutes left, as the striker wriggled brilliantly into space in the box before firing across the goalkeeper and in at the far post.
Little more than 90 seconds later, Alexis Sanchez completed the comeback as he raced into the area and slid under Ionut Radu.
Christophe Pelissier’s side never looked like recovering anything from there, as they slipped to an 11th defeat in 17 away league games.
As for Marseille, they are now just five points behind league leaders PSG with five games remaining, after taking just a second win in seven league outings at the Stade Velodrome.
Flashscore Man of the Match: Cengiz Under (Marseille)
