San Lorenzo's post-Copa Libertadores hangover continued after they were upstaged 2-1 by Argentine Primera Division rivals Defensa y Justicia in their last 16 clash on Wednesday.
Ciro Rius was the hero for the visitors, scoring twice at the Estadio Presidente Juan Domingo Peron as Defensa set up a quarter-final showdown with Atletico Rafaela.
San Lorenzo have struggled since claiming their maiden Copa Libertadores crown last month, winning just two league matches from six outings so far to sit nine points off the pace in 16th spot.
And while they were 2-1 winners over Godoy Cruz at the weekend, San Lorenzo came crashing back to earth as Defensa proved too good.
Defensa hit the front in the 24th minute after some patience build-up play, with Rius controlling the ball inside the penalty area before shooting powerfully across goal and past goalkeeper Leonardo Franco.
Hosts San Lorenzo responded, equalising through Uruguayan striker Martin Cauteruccio in the 38th minute.
After Defensa were dispossessed on halfway, Cauteruccio flicked the ball through the legs of his opponent and fired low into the bottom corner from outside the area.
That was as good as it got for San Lorenzo, who conceded a second goal just six minutes into the second half.
Rius floated in a cross towards the back post and it somehow found the back of the net.
Argentinos Juniors will join Defensa in the quarters after they shocked Racing 1-0 on Thursday.
Former Argentina international Juan Roman Riquelme's 52nd-minute goal was all that separated the two teams as Argentinos - relegated from the top flight last season - advanced to the next round where they will meet Estudiantes.
Racing - ninth in the Primera Division standings - were expected to get the job done, despite back-to-back league defeats.
But Argentinos had other ideas as 36-year-old Riquelme netted the match-winning goal shortly after the break.
