The Copa Libertadores champions progressed to the final of the Fifa-sanctioned tournament with a 2-1 extra-time triumph over Auckland City in Marrakech.
Despite needing 120 minutes to surpass the Oceania Football Confederation champions, midfield veteran Mercier, 34, said his men would not take a backward step against the star-studded Real.
"We know that Real Madrid have got players who are out of this world, who are from a different planet, but we've got things going for us too and we don't bow down to anyone," Mercier told Fifa.com, ahead of the final in Marrakech on Saturday.
"That's what made us the champions of the Americas and that's why we're playing in this tournament. We know what we're here for."
That said, Mercier was taking a moment to appreciate the stage he was about to set foot upon - having previously retired at age 17, before re-starting his career in the fifth tier of Argentine football.
"At my age this is like touching heaven," he said.
"I've played a lot of second division football and reached the top flight at a late age, so I never thought I'd ever be in a situation like this, about to take on Real Madrid.
"I couldn't be more grateful to the game of football."
Mercier, who will be central to San Lorenzo's midfield alongside Nestor Ortigoza, said he and his team-mates experience in battling to the top has only hardened them for such an occasion.
"We've got a lot of seasoned players who've fought all sorts of different battles in both the first and second division, which is really useful for us, on and off the pitch. We're a team that really sticks together. We're united, which is what makes us strong.
He added: "When we're up against a team that is much better than us, we dig as deep as we can and give everything we've got."