Previously unbeaten, Guarani went down 1-0 at Deportivo Capiata – but maintained their three-point lead at the top.
Fabio Escobar haunted his former club, scoring the winner for his fifth-placed side in the 43rd minute.
Libertad moved into second after a 3-1 win over Olimpia, who slipped to third.
Claudio Vargas scored a lucky opener for Libertad as his 35-yard free-kick took a big deflection to find the bottom corner.
The second was even more fortunate as Rodrigo Lopez made it 2-0 in the 37th minute.
An Olimpia defender's attempted clearance looped over his own goalkeeper Victor Centurion and onto the crossbar, allowing Lopez to come in and put away an easy finish.
The visitors got back into the contest seven minutes into the second half as Christian Ovelar found the bottom corner with a header.
Hopes of a comeback were dashed in the 88th minute when Jorge Recalde was left in plenty of space and finished in a one-on-one.
Elsewhere, Roberto Gamarra struck a brace as General Diaz claimed a come-from-behind 2-1 win over Rubio Nu, who finished the clash with 10 men.
Gustavo Santacruz scored a late winner to see 12 de Octubre past Sol de America 3-2.
Sportivo Luqueno's trip to Nacional Asuncion was postponed, as was Cerro Porteno's clash against 3 de Febrero.
