Tournament guests Qatar and 14-times winners Argentina each only have one point after two games and must win on Sunday to stand a chance of staying in the competition.
Elimination from the group stage would be an unbearable humiliation for Argentina and Sanchez, while admitting his side were the underdogs, said they were determined to reach the next round.
"It's true we are not very well known in South America but we have super-professional players and we've come to compete, not to take photos with Messi," Sanchez told a news conference ahead of the game at the Arena Gremio in Porto Alegre.
"They're the favourites and are obliged to win. We'll try to compete against a great team who have the best player in the world. We have a chance to qualify and that's what's exciting, not the fact we could knock out a monster like Argentina."
Messi came back to the Argentina team in March to continue his long quest for a first trophy with his country after taking an extended break following the 2018 World Cup, but his return is not going as planned.
Argentina lost 2-0 to Colombia in their first game and rode their luck in a 1-1 draw with Paraguay as they earned a controversial penalty following a VAR review and goalkeeper Franco Armani saved a spot-kick later on.
Messi scored Argentina's only goal of the tournament by converting from the spot but so far he has not come close to repeating the mesmeric performances he repeatedly delivers for Barcelona.
Sanchez, who led Qatar to a spectacular triumph at the Asian Cup earlier this year, said his side would try to prevent Messi getting the ball around the area but recognised it can be futile to try and stop the five-times world player of the year.
"He can be decisive in any game due to his quality, hundreds of coaches have spent ages trying to stop him but there's no magic formula, it all depends on Leo's feet," added Sanchez, who worked in Barcelona's youth set-up but never coached Messi.
"However much you focus on it it's very difficult to stop him, but we'll try to prevent him from getting involved in the play in the final third of the pitch."
(Reporting by Richard Martin; editing by Tony Lawrence)
