The 45-test Pumas forward was a doubt for the match after spraining his right ankle in last week's 21-16 quarter-final win over the Waikato Chiefs but will now take his place in the second row alongside Tomas Lavanini.
Petti, 24, has been in fine form this year with more lineout wins than any other player in the competition, according to Opta Sports, although Brumbies locks Rory Arnold and Sam Carter are second and third in the rankings.
Prop Enrique Pieretto and flanker Marcos Kremer have also recovered from injuries sustained last week to take their place in the squad but the latter has been relegated to the bench with Tomas Lezana starting at openside.
In the only other change to the squad, Pumas flyer Ramiro Moyano comes onto the bench as outside back cover in place of youngster Santiago Carreras after being sidelined by injury for the last three matches.
The Argentine side, who won the South African conference, will be playing their first Super Rugby semi-final against the twice champion Canberra-based Brumbies at the Jose Amalfitani Stadium.
Nine-times champions the Canterbury Crusaders take on the Wellington Hurricanes in the other, all-New Zealand, semi-final in Christchurch on Saturday.
Jaguares: 15-Emiliano Boffelli, 14-Sebastian Cancelliere, 13-Matias Orlando, 12-Jeronimo de la Fuente (captain), 11-Matias Moroni, 10-Joaquin Diaz Bonilla, 9-Tomas Cubelli, 8-Javier Ortega Desio, 7-Tomas Lezana, 6-Pablo Matera, 5-Tomas Lavanini, 4-Guido Petti, 3-Santiago Medrano, 2-Agustin Creevy, 1-Mayco Vivas
Replacements: 16-Julian Montoya, 17-Nahuel Tetaz Chaparro, 18-Enrique Pieretto, 19-Marcos Kremer, 20-Francisco Gorrissen, 21-Felipe Ezcurra, 22-Domingo Miotti, 23-Ramiro Moyano
(Writing by Nick Mulvenney in Sydney, editing by Ian Ransom)
