Led by chef d'equipe Mark Laskin of Canada, the Mexican squad topped the leaderboard ahead of Sweden and Brazil at the Real Club de Polo.
Eight teams will advance to Sunday's final competition in Barcelona; Canada is eliminated in the first round.
Brazil goes first at the Final in Barcelona; Team Canada's chef d'equipe is Eric Lamaze, while Mark Laskin leads Team Mexico.
The Canadian Team will be comprised of Beth Underhill, Tim Wilks, Rebecca McGoldrick and Alicia Gadban-Lewis.
Course designer Uliano Vezzani’s difficult jumping track had a significant influence on the final placings, allowing just 12 clear rounds.
Michael Jung sits atop the leaderboard with fischerChipmunk going into the final day at the FEI Eventing World Championship.
As the World Eventing Championships dressage wrapped up in Pratoni, the German rider snagged the individual lead ahead of cross-country day.
Great Britain moves into the overnight lead with strong performances on a blustery dressage day that some horses found a bit too exciting.
With Olympic gold medalists and the 2018 WEG individual champion among the 90 riders from 27 countries, it will be an epic competition.
A double-clear effort and a blazing fast final round helped clinch the Nations Cup victory for the Irish team over France.