Pumas head coach Marcelo Loffreda surprisingly included the Cordoba Athletic player in the 41-man preliminary list but Fessia is taking nothing for granted with stiff competition from Martin Schusterman and Lucas Ostiglia.
Fessia told www.ole.com.ar: "They (Schusterman and Ostiglia) don't have to fight for the place in the official list.
"I have to rob it from one of them and I have to take the place by merit. I'm the newcomer and that's a disadvantage for me."
Fessia believes other back row berths are already filled, saying: "I have no doubts (Martin) Durand, Juan Fernandez Lobbe, (Gonzalo) Longo and Juan Leguizamon are in the World Cup squad."
Fessia added that he felt shocked when Loffreda told him he was in the list that will start a two-week training camp in Pensacola, United States, next week.
"I was very surprised, but delighted, obviously," said the 25-year-old. "I'm happy to be in this situation.
"I'm new here and the coach picked me up instead of some other more experienced players and I'm very happy about that."
Fessia is taking things one step at a time, saying: "In Pensacola, I want to meet the rest of the squad.
"Then I want to enjoy the experience, because to be in a World Cup pre-squad is not usual.
"I'm not thinking about the World Cup. I am only thinking about the trip to the United States and to do things well there."